Electrochemical properties of lithium–titanium oxide, modified with Ag–Cu particles, as a negative electrode for lithium-ion batteries

Abstract

Composites of Li₄Ti₅O₁₂ with Ag–Cu particles were successfully synthesized by solid-state reaction followed by thermal decomposition of the metal substrates. The presence of metallic particles was confirmed by X-ray diffraction, scanning transmission electron microscopy and X-ray photoelectron spectroscopy. Galvanostatic charge–discharge tests showed improved specific capacity and capacity retention of Li₄Ti₅O₁₂/Ag–Cu composites at a 10C current rate, while cyclic voltammetry and electrochemical impedance spectroscopy revealed changes in Li⁺ ion chemical diffusion coefficient values and charge-transfer resistance with increasing amount of Ag–Cu in prepared powders. The synthesis and structural, morphological and electrochemical evaluation of Li₄Ti₅O₁₂/Ag–Cu powders, carried out in this work, were also presented here for the first time.

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1. Introduction

One of the candidates to replace graphite as an anode material in lithium-ion batteries is lithium–titanium oxide (Li₄Ti₅O₁₂, LTO) of spinel structure. It shows excellent cyclability due to negligible volume change during intercalation/deintercalation reactions (so called “zero-strain” electrode material).^1,2 A high operating potential of 1.55 V vs. Li⁺/Li⁰ is responsible for LTO's enhanced safety, due to hindering the formation of a Solid Electrolyte Interface (SEI) layer on the electrode's surface, although some research groups reported SEI creation as a result of spontaneous reactions with the electrolyte or migration of SEI components formed on counter electrodes.^3,4 Li₄Ti₅O₁₂ has an acceptable specific capacity of 175 mAh g⁻¹ when charged/discharged in the potential range of 1.00–3.00 V (vs. Li⁺/Li⁰) when the following reaction occurs:

Li₄Ti₅O₁₂ + 3Li⁺ + 3e⁻ ↔ Li₇Ti₅O₁₂, (1)

and 293 mAh g⁻¹ while expanding the potential window to 0.01–3.00 V (vs. Li⁺/Li⁰). Increased capacity of lithium–titanium oxide when cycled to 0.01 V is related to full reduction of titanium ions from +4 to +3 oxidation state which involves additional two Li⁺ ions:⁵

Li₇Ti₅O₁₂ + 2Li⁺ + 2e⁻ ↔ Li₉Ti₅O₁₂. (2)

All mentioned properties make LTO a good material for next generations of Li-ion batteries. However, due to low electric conductivity of Li₄Ti₅O₁₂, its widespread application is still limited.^6–8

Many approaches were suggested to improve the electrochemical performance of electrode materials for lithium-ion batteries which included, among others, carbon coating,⁹ ion doping,¹⁰ surface modifying^11,12 or even electrode design.¹³ In Li₄Ti₅O₁₂ case, to enhance its electrochemical properties, many research groups tried various synthetic routes (such as solid-state,^14–18 sol–gel,^19–23 hydrothermal,^24–28 combustion,^29–31 microwave,^32–34 spray pyrolysis,^35,36 or atomic layer deposition³⁷), cation (Mg²⁺,^38,39 Ni²⁺,⁴⁰ Al³⁺,^41–43 Cr³⁺,^44,45 Zr⁴⁺,^46–48 V⁵⁺,^49,50 Nb⁵⁺,⁵¹ Ta⁵⁺,⁵²) and anion (Br⁻,^53,54 F⁻,^41,55,56) doping or surface modification with conductive materials (carbon coat,^57–61 graphene/graphene oxide,^62–65 carbon nanotubes,^66–68 Ag,^69–79 Cu,^80–84 Au,⁸⁵ TiN^86–88).

In our previous studies, we synthesized and fully characterized Li₄Ti₅O₁₂ modified with silver nanoparticles.^78,79 The deposition of Ag nanoparticles highly enhanced electrochemical properties of lithium–titanium oxide, such as cyclability, high-rate performance and intercalation/deintercalation reaction kinetics. Due to the fact, that silver is considered as an “expensive” metal, in this work we focused on reducing the amount of Ag by incorporating a metal from the same periodic table's group into LTO powders, which is less costly and highly conductive, obtaining and fully characterizing a cheaper, silver–copper composite of Li₄Ti₅O₁₂ with enhanced electrochemical properties in lithium-ion batteries.

2. Experimental

2.1 Synthesis of pristine Li₄Ti₅O₁₂

The one-step solid-state synthesis was used to obtain single-phase lithium–titanium oxide powder of spinel structure. The synthesis was carried out under high-energy ball-milling (HEBM) process, using ethanol as a medium and zirconia balls, in a Planetary Mono Mill PULVERISETTE 6 (Fritsch, Germany). The starting reagents used were titanium dioxide (TiO₂, 99%, Sigma-Aldrich) mixed with lithium carbonate (Li₂CO₃, synthesized in-house, at the Institute of Electronic Materials Technology) at the Ti : Li molar ratio of 5 : 4. The high-energy ball-milling process was carried out at a constant speed of 200 rpm and processing time of 12 h, respectively. Following the ball-milling process, the remaining alcohol medium was evaporated and the obtained powder was subsequently dried at 150 °C for a few hours in air atmosphere. Finally, the powder was ground in an agate mortar and heat-treated in air: a two-stage thermal treatment, consisting of 6 h annealing at 500 °C followed by 20 h annealing at 800 °C, was employed.¹⁸

2.2 Synthesis of Li₄Ti₅O₁₂/Ag–Cu composites

To synthesize the Li₄Ti₅O₁₂/Ag–Cu composites, AgNO₃ (Avantor Performance Materials Poland S.A.) and Cu(NO₃)₂·3H₂O (Chempur) were firstly dissolved in an ethanol solution, separately. The nitrate–ethanol solutions of Ag and Cu were mixed together. Then, the prepared nanocrystalline Li₄Ti₅O₁₂ powder was added to the silver–copper nitrate solution to obtain a suspension. In subsequent processes the weight ratio of Ag : Cu (equal volume) and LTO was 0.005 : 0.005, 0.01 : 0.01, 0.015 : 0.015, 0.02 : 0.02 and 0.025 : 0.025, respectively. The mixture was magnetically stirred for a few hours to obtain a homogenously dispersed suspension and then was dried in air for a few hours at 150 °C. At the last step, the composites were ground in an agate mortar to obtain a fine powder.^78,79

2.3 XRD, SEM, STEM, XPS

Phase identification of the prepared samples was carried out by X-ray diffraction (XRD) using an X-ray Powder Diffractometer Siemens D-500. X-ray profiles were measured between 10 and 60° (2θ angle) with a Cu Kα radiation source (λ = 1.542 Å).

Scanning electron microscopy (SEM) was used to study the microstructure of the surface modified LTO powders. Samples were prepared by dispersing a small portion of dry powders on a conductive carbon tape and were investigated on Hitachi S5500 (Hitachi High Technologies Corporation, Japan) scanning electron microscope using backscattered electron (BSE) detector. Scanning transmission electron microscope Hitachi-HD2700 (STEM), operated at 200 kV and equipped with energy dispersive X-ray spectrometer (EDS) was used for more detailed microstructural analysis. Bright Field STEM and High Angle Annular Dark Field STEM modes were used for imaging.

X-ray photoelectron spectroscopy (XPS) experiments have been performed using Kratos Axis Supra instrument with Al Kα monochromatic beam (1486.7 eV) as an X-ray source. The X-ray takeoff angle was set to standard 45°. Pass energy was set to 160 eV and 20 eV for low and high resolution spectra acquisition respectively, providing an energy resolution of ∼7.5 and ∼0.8 eV respectively. Data treatment was performed within the ESCA software. Curve fitting was achieved by using standard Shirley background subtraction and Gaussian/Lorentzian peak shapes. All spectra were calibrated by using C 1s adventitious carbon as a reference binding energy (284.8 eV).

2.4 Electrochemical measurements

For electrochemical measurements, the electrodes were made of Li₄Ti₅O₁₂/Ag–Cu composite ground with Vulcan XC72R (Cabot) carbon in an agate mortar for 20 min. The obtained powder was added to 5% solution of polyvinylidene fluoride (Alfa Aesar) in N-methyl pyrrolidinone (Sigma-Aldrich) and the mixture was homogeneously stirred for 4 h. Such prepared slurry was uniformly coated onto copper foil using Elcometer® 3545 and dried in 55 °C for 1 h in air and 120 °C overnight in vacuum. Round electrodes were then cut from the foil and pressed in hydraulic press under 200 bar pressure for 1 min followed by vacuum drying at 120 °C for 16 h. The electrode composition was 8 : 1 : 1 wt ratio of Li₄Ti₅O₁₂/Ag–Cu : PVdF : carbon. The cells were assembled in an argon-filled glove-box (MBraun Unilab MB-20-G).

The electrochemical tests were carried out in three-electrode Swagelok®-type cells with Li₄Ti₅O₁₂/Ag–Cu working electrode, lithium metal foil (Sigma-Aldrich) as a reference and a counter electrodes, a porous polymer separator (Celgard® 2400) and liquid electrolyte mixture containing 1 M LiPF₆ in ethylene and dimethyl carbonate (1 : 1 wt/wt) solvents (BASF). The chronopotentiometry (CP) experiments were performed using a multichannel battery tester Sollich ATLAS 1361. All examined cells were preliminarily discharged at current rate of 0.1C (C corresponds to current density of 175 mA g⁻¹). Then, the cyclability and high-rate tests were performed. For cyclability tests, the cells were charged/discharged at constant current rate of 1C for 50 consequent cycles. For high-rate tests, the cells were charged at constant current rate of 1C and were discharged at current rates of 1, 2, 5 and 10C for 5 cycles each. All the cells were cycled in 1.00–3.00 V potential range (vs. Li⁺/Li⁰). After high-rate tests, the cells were examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using CHI760d electrochemical workstation. The CV measurements were performed in the potential range of 1.00–2.20 V (vs. Li⁺/Li⁰) at scan rates of 0.1, 0.2, 0.5, 1.0 and 2.0 mV s⁻¹. The EIS experiments were performed at the potential of 1.00 V (after one hour of cell conditioning) with the amplitude of 10 mV in 10⁻²–10⁵ Hz frequency range. The EIS spectra were analyzed using ZView2 software. For the lithium ion diffusion coefficient calculations, for both CV and EIS tests, the geometric surface area of the electrodes was used.

3. Results and discussion

3.1 XRD, SEM, STEM, XPS

Fig. 1 depicts the XRD patterns of the Ag–Cu modified Li₄Ti₅O₁₂ powders. All the peaks from each sample associated with lithium–titanium oxide are in good agreement with the spinel phase Li₄Ti₅O₁₂ pattern (ICDD-49-0207) confirming, that modifying the surface using Ag–Cu nanoparticles of pristine Li₄Ti₅O₁₂ does not lead to any phase segregation. As presented in Fig. 1 all seven characteristic XRD peaks observed for Li₄Ti₅O₁₂/Ag–Cu powders at the 2θ angles of: 18.35, 30.21, 35.60, 27.24, 43.28, 47.39 and 57.27° correspond to the (111), (220), (311), (222), (400), (331) and (333) crystal planes of the cubic structure of lithium–titanium oxide (ICDD-49-0207), respectively. The additional 2 peaks (signed as “+” in Fig. 1) at 38.11 and 44.28° correspond to the (111) and (200) planes of cubic structure of metallic silver (silver-3C, syn, cubic, Fm3m; ICDD-04-0783), respectively. The intensity of these two reflections increased with the increasing of Ag (and also Cu) content in all Li₄Ti₅O₁₂/Ag–Cu powders. Also, it should be pointed out, that the characteristic reflection line of Cu metallic at 43.29° (intensity of 100%, at (111) plane) matched to reflection line of LTO (43.28°; ICDD-04-0836). A systematic appearance of, mentioned above, two characteristics diffraction lines for metallic silver indicated that metallic Ag nanoparticles were successfully deposited on the surface of Li₄Ti₅O₁₂ (as we presented in our previous work).⁷⁶ It should be noted, that the process of surface modification of LTO grains using both silver and copper, nanoparticles in equal volume was reported in this work for the first time.

Fig. 1 XRD patterns of Li₄Ti₅O₁₂/Ag–Cu composites (* – Li₄Ti₅O₁₂, + – Ag).

Lattice parameters of the lithium titanium oxide powders and their average crystallite sizes, calculated based on the XRD patterns shown in Fig. 1, are listed in Table 1. No significant change related to Ag–Cu presence was observed. In particular, for all samples, the average crystallites sizes of lithium–titanium oxide (as calculated from Scherrer formula, allowing for the instrumental line broadening) were in the range of 64 to 90 nm. The unit cell parameter of Li₄Ti₅O₁₂ powders with silver–copper nanoparticles calculated from the XRD data is consistent with the standard value (a₀ = 8.359 Å) of the ICDD PDF card.

Table 1 Crystallite sizes and lattice parameters for Li₄Ti₅O₁₂/Ag–Cu composites obtained from XRD measurements

Ag–Cu content [%]	Average crystallite sizes [nm] (±5 nm)	Lattice parameter [Å] (±0.001 Å)
1	81	8.358
2	90	8.359
3	85	8.357
4	81	8.357
5	64	8.354

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The SEM images of Li₄Ti₅O₁₂/Ag–Cu composite powders are presented in Fig. 2. The Ag–Cu particles are visible on BSE images as bright particles located on lithium–titanium oxide substrate. It can be seen, that the Ag–Cu nanoparticles are below 100 nm in diameter and they are distributed on LTO surface relatively homogeneous. The SEM analysis did not show significant difference in Ag–Cu nanoparticles size for lower concentration (1–3%) and slight growth of the nanoparticles size for higher concentration (4–5%). In this case Ag–Cu nanoparticles with diameter above 100 nm can be observed.

Fig. 2 SEM images of Li₄Ti₅O₁₂/Ag–Cu composites: (A) 0%, (B) 1%, (C) 2%, (D) 3%, (E) 4%, (F) 5% wt Ag–Cu.

STEM images confirms homogeneous distribution Ag–Cu particles on LTO surface (Fig. 3A and B). The differences in brightness intensity observed for different areas of Ag–Cu particles on HAADF-STEM images suggest elemental composition variability (Fig. 3B). It means that Ag–Cu particles composition is not homogenous. This observations were confirmed by EDS mapping results where silver content was found only in selected areas of Ag–Cu particles (Fig. 3C and D). The example of EDS spectrum taken from Ag–Cu particle area, indicated by the circle, is presented in Fig. 3E. The Cu-L and Ag-L lines, visible on the spectrum, confirm the presence of silver and copper elements in one particle. Due to small volume of analyzed material, the signal is relatively weak, however the peaks of Ag and Cu are clearly visible. Other observed peaks are related to oxygen from Li₄Ti₅O₁₂ particle and sample contamination.

Fig. 3 STEM analysis of Li₄Ti₅O₁₂/5% Ag–Cu composite: (a) STEM, (b) HAADF-STEM, (c) Cu EDS mapping, (d) Ag EDS mapping, (e) EDS spectrum of the area circled in red.

Fig. 4 represents the XPS wide scan spectrum of Li₄Ti₅O₁₂/5% Ag–Cu powder. It also includes wide scan of Li₄Ti₅O₁₂/5% n-Ag sample prepared by the same method and taken as a reference material.⁷⁸ The spectrum comprises several main peaks which correspond to titanium, oxygen and silver. Spectrum of Li₄Ti₅O₁₂/5% Ag–Cu also reveals the presence of copper in the sample: Cu 2p_3/2 and Cu 2p_1/2 peaks at 932.6 eV and 952.0 eV binding energies respectively. It can be also seen that Li₄Ti₅O₁₂/5% n-Ag powder is slightly contaminated by nitrogen (from AgNO₃ precursor) as well as zirconium and silicon (from ceramic mortar). The relative surface concentrations of main species present in both samples are listed in Table 2. Lithium–titanium oxide modified with 5% wt of silver nanoparticles reveals a significant amount of Ag at the surface of grains (21% wt). In comparison, surface concentration of silver in LTO sample modified with 2.5% wt of silver and 2.5% wt of copper is much lower (6% wt). It might indicates that silver in Li₄Ti₅O₁₂/5% Ag–Cu powder forms much larger grains of non-uniform distribution resulting in apparently lower concentration of Ag detected by XPS measurement. This statement is consisted with EDS mapping, which confirms high agglomeration of silver in Li₄Ti₅O₁₂/5% Ag–Cu sample. One can also recognize that surface copper content is 33% lower than silver even though bulk concentration of these specie is equal. It suggest that Ag localizes on the surface of Cu. This observations are consistent with EDS mapping. The presence of the most intense peaks (Ag 3d located at ca. 370 eV) at the XPS spectra is a clear evidence of silver existence at the surface of examined material. It can be seen, that spectra of Li₄Ti₅O₁₂/n-Ag sample consist of mainly Ag 3s, Ag 3p and Ag 3d photoelectron lines, although, due to decreasing of cross-section factor, the Ag 3s and Ag 3p peaks are of diminished intensity, in comparison with Ag 3d peak. In the case of Li₄Ti₅O₁₂/Ag–Cu sample, the surface concentration of silver is much lower, thus the Ag 3s and Ag 3p peaks are barely detected.

Fig. 4 XPS spectra of Li₄Ti₅O₁₂/5% Ag–Cu and Li₄Ti₅O₁₂/5% n-Ag composites.

Table 2 Mass surface concentrations of titanium, oxygen, silver and copper elements in Li₄Ti₅O₁₂/5% n-Ag and Li₄Ti₅O₁₂/5% Ag–Cu composites

Element	Mass surface concentration [%]
	5% n-Ag	5% Ag–Cu
Ti	35	48
O	44	44
Ag	21	6
Cu	—	2

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3.2 Electrochemistry

3.2.1 Cyclability tests. Fig. 5 presents the initial discharge curves of pristine and Ag–Cu modified Li₄Ti₅O₁₂ during cyclability tests. All examined samples showed higher specific capacity than theoretical value of 175 mAh g⁻¹ for lithium–titanium oxide during discharge at 0.1C rate. The extra charge is related to non-intercalation processes, involving electrode activation, reduction of trace water present in the electrolyte²⁸ or corrosion of copper current collector.^89,90 Fig. 6 and 7 presents the results of cyclability tests. Charge/discharge curves (Fig. 6) show that the insertion/extraction of Li⁺ ions into/out of Li₄Ti₅O₁₂ framework occurs at a stable potential plateau of 1.58/1.54 V (vs. Li⁺/Li⁰) during charge and discharge, respectively. No other electrochemical processes were present in examined cells. Ag–Cu modification had no impact on specific capacity of Li₄Ti₅O₁₂ composite powders at 1C rate and all examined electrodes' specific capacity was in 160–162 mAh g⁻¹ range. All studied powders also showed good cyclability (Fig. 7), maintaining between 92 and 96% specific capacity retention after 50 charge/discharge cycles. No dependency between cyclability of Li₄Ti₅O₁₂ and the amount of mixed Ag–Cu modification in examined powders was found. The results of cyclability tests were also listed in Table 3.

Fig. 5 Initial discharge curves of Li₄Ti₅O₁₂/Ag–Cu composites during cyclability tests.

Fig. 6 Charge/discharge curves for Li₄Ti₅O₁₂/Ag–Cu composites during cyclability tests: solid line – 1^st cycle, dashed line – 50^th cycle.

Fig. 7 Cyclability of Li₄Ti₅O₁₂/Ag–Cu composites.

Table 3 Results of cyclability CP tests of Li₄Ti₅O₁₂/Ag–Cu composites

Material	Electrode loading [mg cm^−2] (±0.02 mg cm^−2)	Initial discharge capacity [mAh g^−1]	1st discharge capacity [mAh g^−1]	50th discharge capacity [mAh g^−1]	1st charge capacity [mAh g^−1]	50th charge capacity [mAh g^−1]	Capacity retained after 50th cycle [%] (±0.01%)
Pristine	2.19	193.5 ± 1.7	161.9 ± 1.4	151.9 ± 1.4	167.9 ± 1.5	151.2 ± 1.3	93.84
1% Ag–Cu	1.92	187.6 ± 1.9	161.1 ± 1.6	153.1 ± 1.6	166.1 ± 1.7	153.0 ± 1.6	94.99
2% Ag–Cu	2.05	211.2 ± 2.0	161.9 ± 1.6	149.7 ± 1.4	168.0 ± 1.6	148.8 ± 1.4	92.50
3% Ag–Cu	2.14	194.0 ± 1.8	161.0 ± 1.5	148.3 ± 1.4	166.9 ± 1.5	148.1 ± 1.4	92.16
4% Ag–Cu	2.29	189.9 ± 1.6	160.0 ± 1.4	153.8 ± 1.3	166.3 ± 1.4	153.3 ± 1.3	96.08
5% Ag–Cu	2.13	198.9 ± 1.8	160.2 ± 1.5	151.6 ± 1.4	166.9 ± 1.5	151.3 ± 1.4	94.62

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3.2.2 High-rate tests. Fig. 8 shows the initial discharge curves of Li₄Ti₅O₁₂/Ag–Cu composites during high-rate tests. Similar to cyclability tests, the examined cells showed higher specific capacity than theoretical value for LTO, suggesting other, non-faradaic processes present during the initial discharge. The results of high-rate tests are presented on Fig. 9, 10 and in Table 4. Also in this case, the examined electrodes' specific capacity at 1C rate was in 160–162 mAh g⁻¹ range and no relationship between specific capacity at 1C rate and the amount of Ag–Cu modification was found. However, after raising the current rate, Li₄Ti₅O₁₂/Ag–Cu powders behavior began to differ from each other. At 10C rate, every modified sample reveals higher specific capacity than pristine one, reaching 101.1 ± 1.0 mAh g⁻¹ for Li₄Ti₅O₁₂/5% Ag–Cu composite and retaining 63.13 ± 0.01% of specific capacity at 1C rate. On the other hand, the pristine sample showed only 82.0 ± 0.7 mAh g⁻¹ of specific capacity and retained 50.59 ± 0.01% of its 1C rate specific capacity while discharged at 10C current rate. From Fig. 10, one can also see, that every modified sample has higher operating potential than pristine powder. All the above suggest better reaction kinetics and lower electrodes polarization for Li₄Ti₅O₁₂/Ag–Cu compounds. In order to get a better insight on the reasons behind enhanced electrochemical properties of Ag–Cu modified composite powders we conducted CV and EIS experiments.

Fig. 8 Initial discharge curves of Li₄Ti₅O₁₂/Ag–Cu composites during high-rate tests.

Fig. 9 Rate capability of Li₄Ti₅O₁₂/Ag–Cu composites.

Fig. 10 Discharge curves of Li₄Ti₅O₁₂/Ag–Cu composites at 10C rate.

Table 4 Results of high-rate CP tests of Li₄Ti₅O₁₂/Ag–Cu composites

Material	Electrode loading [mg cm^−2] (±0.02 mg cm^−2)	Initial discharge capacity [mAh g^−1]	1C discharge capacity [mAh g^−1]	2C discharge capacity [mAh g^−1]	5C discharge capacity [mAh g^−1]	10C discharge capacity [mAh g^−1]	Capacity retained at 10C [%] (±0.01%)
Pristine	2.16	200.3 ± 1.8	162.1 ± 1.5	136.0 ± 1.2	105.9 ± 1.0	82.0 ± 0.7	50.59
1% Ag–Cu	2.27	196.8 ± 1.7	160.9 ± 1.4	147.8 ± 1.3	118.9 ± 1.0	88.4 ± 0.8	54.95
2% Ag–Cu	1.99	206.9 ± 2.0	162.3 ± 1.6	150.3 ± 1.5	125.3 ± 1.2	97.0 ± 1.0	59.76
3% Ag–Cu	2.28	200.2 ± 1.7	161.1 ± 1.4	150.1 ± 1.3	125.7 ± 1.1	98.4 ± 0.8	61.12
4% Ag–Cu	2.15	203.6 ± 1.9	160.5 ± 1.5	150.8 ± 1.4	126.9 ± 1.2	99.7 ± 0.9	62.11
5% Ag–Cu	1.98	195.4 ± 1.9	160.1 ± 1.6	148.0 ± 1.5	125.9 ± 1.2	101.1 ± 1.0	63.13

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3.2.3 Cyclic voltammetry. Fig. 11 presents CV curves of pristine and Ag–Cu modified Li₄Ti₅O₁₂. One can see, that the peaks originated only from oxidation and reduction reactions of Li₄Ti₅O₁₂ and no other signals are visible. With increasing amount of Ag–Cu in analyzed composite powders, one can see slight changes in CV curves resulting in narrowed peak shape. This phenomenon is a result of better reaction kinetics and lower electrode's resistance after surface modification.⁹¹ Increasing the scan rate resulted in higher peak currents and increase in oxidation–reduction peak separation (caused mainly by electrode's polarization). The linear trend of relationship between current intensity and square root of scan rate had been present in all examined samples (Fig. 12), which suggests a diffusion-controlled mechanism of oxidation/reduction reactions. According to Randles–Sevcik equation for a reversible electrochemical process, I_p(v^1/2) relationship can be described as:^92,93in which A is electrode's electrochemically active surface area (cm²), v is scan rate (V s⁻¹), n is number of electrons involved during the electrochemical process, F is Faraday's constant (C mol⁻¹), R is gas constant (J (mol K)⁻¹), T is absolute temperature (K), C is molar concentration of lithium ions in Li₄Ti₅O₁₂ crystal (mol cm⁻³) and D is chemical diffusion coefficient of lithium ions inside Li₄Ti₅O₁₂ matrix (cm² s⁻¹), described as:and dependent on both Li⁺ ion and electron diffusion in the host matrix.⁹¹ By using the eqn (3) and equations obtained from linear functions of I_p(v^1/2), we calculated Li⁺ ion chemical diffusion coefficient values for oxidation and reduction reactions of Li₄Ti₅O₁₂/Ag–Cu powders (Fig. 13) and listed them in Table 5. Because of the two-phase mechanism of LTO intercalation/deintercalation reaction and poor Li⁺ ionic conductivity of fully reduced compound (Li₇Ti₅O₁₂ of rock-salt structure),⁹⁴ Li⁺ ion transport across lithium–titanium oxide matrix is faster during oxidation reaction, which corresponds to higher Li⁺ chemical diffusion coefficient values for oxidation processes.

Fig. 11 CV curves of Li₄Ti₅O₁₂/Ag–Cu composites: (A) 0%, (B) 1%, (C) 2%, (D) 3%, (E) 4%, (F) 5% wt Ag–Cu.

Fig. 12 I_p(v^−1/2) relationship acquired from CV curves of Li₄Ti₅O₁₂/Ag–Cu composites.

Fig. 13 Li⁺ chemical diffusion coefficient values calculated from CV experiments of Li₄Ti₅O₁₂/Ag–Cu composites.

Table 5 Li⁺ chemical diffusion coefficient values calculated from CV experiments of Li₄Ti₅O₁₂/Ag–Cu composites

Material	Electrode loading [mg cm^−2] (±0.02 mg cm^−2)	DOX [cm^2 s^−1]	DRED [cm^2 s^−1]
Pristine	2.13	(4.31 ± 0.11) × 10^−10	(1.88 ± 0.08) × 10^−10
1% Ag–Cu	2.35	(5.26 ± 0.26) × 10^−10	(2.66 ± 0.22) × 10^−10
2% Ag–Cu	2.08	(6.54 ± 0.48) × 10^−10	(3.26 ± 0.15) × 10^−10
3% Ag–Cu	2.26	(6.11 ± 0.18) × 10^−10	(3.28 ± 0.17) × 10^−10
4% Ag–Cu	2.19	(8.34 ± 0.48) × 10^−10	(4.14 ± 0.10) × 10^−10
5% Ag–Cu	1.98	(9.41 ± 0.73) × 10^−10	(4.33 ± 0.13) × 10^−10

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Modification of Li₄Ti₅O₁₂ surface resulted in an increase in chemical diffusion coefficient values for every modified sample – from (4.31 ± 0.11) × 10⁻¹⁰ and (1.88 ± 0.08) × 10⁻¹⁰ cm² s⁻¹ for pristine LTO oxidation and reduction processes to (9.41 ± 0.73) × 10⁻¹⁰ and (4.33 ± 0.13) × 10⁻¹⁰ cm² s⁻¹ for Li₄Ti₅O₁₂/5% Ag–Cu oxidation and reduction reactions. Furthermore, with every increase of Ag–Cu amount in examined powders the values of D increased. There is no saturation in chemical diffusion coefficients occurring while increasing the amount of Ag–Cu modification, which suggest that the lithium–titanium oxide's particles are not fully covered with silver and copper even at 5% wt of Ag–Cu. These findings are in good agreement with SEM, XPS, galvanostatic measurements and with our previous studies about Li₄Ti₅O₁₂/n-Ag composites^78,79 and can be explained by increase in electrode's active surface area and the amount of active centers capable of insertion/extraction reaction of Li⁺ ions into/out of Li₄Ti₅O₁₂ framework⁷⁹ but, as the LTO's surface is not covered completely by silver–copper particles, the electrochemical performance of Li₄Ti₅O₁₂/Ag–Cu composites is not as good as in the case of Li₄Ti₅O₁₂/n-Ag⁷⁸ due to lower electrochemically active surface area of examined composites than in our previous studies.⁷⁹

3.2.4 Electrochemical impedance spectroscopy. Fig. 14 presents the complex plane plots of pristine and Ag–Cu modified lithium–titanium oxide. The equivalent circuit used for data fitting is presented in the inset of Fig. 14. In the mentioned circuit, R_S corresponds to ohmic resistance (intersection with Z′ axis at high frequency), R_SEI is the resistance of passivation film on electrode's surface (depressed semicircle at high frequencies), R_CT is the charge-transfer resistance through electrode|electrolyte interface (depressed semicircle at medium frequencies), CPE elements correspond to capacitance of Li⁺ transport through SEI layer and electrode's surface and W_O is a Warburg resistance linked to a long-range diffusion of Li⁺ ions inside lithium–titanium oxide crystal (inclined line at low frequencies).^4,54–56,75 The results obtained through data fitting are listed in Table 6. One can see, that Ag–Cu modification resulted in lower R_CT values of Li₄Ti₅O₁₂/Ag–Cu composites, suggesting faster reaction kinetics and increased conductivity of synthesized composite powders. After reaching 3% wt of Ag–Cu modification the charge-transfer resistance stabilizes in the range of 17–25 Ω, which is ca. 30 Ω lower than for pristine sample.

Fig. 14 Complex plane plots of Li₄Ti₅O₁₂/Ag–Cu composites. Inset shows the equivalent circuit used for data fitting.

Table 6 Data acquired after fitting and analysis of EIS spectra of Li₄Ti₅O₁₂/Ag–Cu composites

Material	Electrode loading [mg cm^−2] (±0.02 mg cm^−2)	RS [Ω]	RSEI [Ω]	RCT [Ω]	DEIS [cm^2 s^−1]
Pristine	2.13	2.82 ± 0.05	14.26 ± 0.19	58.78 ± 2.60	(6.20 ± 0.69) × 10^−14
1% Ag–Cu	2.25	3.47 ± 0.03	12.67 ± 0.19	62.81 ± 2.60	(1.12 ± 0.11) × 10^−13
2% Ag–Cu	2.08	3.08 ± 0.03	14.52 ± 0.18	50.57 ± 2.02	(1.31 ± 0.14) × 10^−13
3% Ag–Cu	2.23	2.36 ± 0.02	6.57 ± 0.14	24.48 ± 1.85	(1.64 ± 0.16) × 10^−13
4% Ag–Cu	2.19	2.34 ± 0.06	16.09 ± 0.21	17.27 ± 1.24	(1.98 ± 0.10) × 10^−13
5% Ag–Cu	1.91	3.13 ± 0.04	9.72 ± 0.13	22.03 ± 1.20	(2.53 ± 0.27) × 10^−13

---

We calculated Li⁺ ion chemical diffusion coefficient from EIS data by using the equation:^{13,15,25,26,34,47}

in which σ is a Warburg coefficient (Ω s⁻¹), corresponding to a slope of Z′(ω^−1/2) function, described by equation:

Z′ = const + σω^−1/2, (6)

and fulfilled for the frequency range in which the EIS spectrum is inclined to Z′ axis at 45° (Fig. 15). The rest of the symbols have the same meaning as in eqn (3). The acquired D values are presented on Fig. 16 and listed in Table 6. The obtained lithium ion chemical diffusion coefficient values show similar trend with increasing Ag–Cu content in Li₄Ti₅O₁₂/Ag–Cu composite as in the case of CV experiments. One can see an increase in D values with increasing amount of Ag–Cu modification – from (6.20 ± 0.69) × 10⁻¹⁴ cm² s⁻¹ for pristine powder to (2.53 ± 0.27) × 10⁻¹³ cm² s⁻¹ for Li₄Ti₅O₁₂/Ag–Cu composite. EIS results are also in good agreement with galvanostatic tests and with our previous studies about Li₄Ti₅O₁₂/n-Ag compounds^78,79 and can be explained by enhancing the surface conductivity of Li₄Ti₅O₁₂, enhancing its electrochemically active surface area and providing more active centers for intercalation/deintercalation reactions to take place.⁷⁹ Also in this case, the saturation of D values is not present, which suggest not fully covered crystals of Li₄Ti₅O₁₂ by Ag–Cu modification, as seen on SEM and XPS results, and lower electrochemically active surface area than in our previous studies about Li₄Ti₅O₁₂/n-Ag compounds.⁷⁹

Fig. 15 Z′(ω^−1/2) relationship acquired from EIS experiments of Li₄Ti₅O₁₂/Ag–Cu composites.

Fig. 16 Li⁺ chemical diffusion coefficient values calculated from EIS spectra of Li₄Ti₅O₁₂/Ag–Cu composites.

4. Conclusions

We successfully synthesized lithium–titanium oxide and modified its surface with Ag–Cu metals. The XRD and SEM experiments showed a presence of metallic nanoparticles, 100 nm in diameter, in examined powders. XPS experiments revealed lower surface concentration of Ag–Cu particles than in the case of Li₄Ti₅O₁₂/5% n-Ag composite. Galvanostatic charge/discharge evaluation of studied materials showed enhanced electrochemical properties of Li₄Ti₅O₁₂ powders after Ag–Cu modification, namely ca. 12.5% increase in specific capacity retention at 10C current rate and ca. 19 mAh g⁻¹ increase in specific capacity at 10C current rate were observed. Furthermore, CV and EIS experiments showed an increase in Li⁺ chemical diffusion coefficient values with increasing amount of Ag–Cu present in Li₄Ti₅O₁₂/Ag–Cu composites and decrease in examined powders impedance and charge-transfer resistance after Ag–Cu surface modification.

Enhanced electrochemical properties of Li₄Ti₅O₁₂/Ag–Cu composites are an effect of Li₄Ti₅O₁₂ surface activation through contact with conductive particles. That mechanism was more thoroughly explained in our previous work.⁷⁹ This research is in very good agreement with our previous studies about Li₄Ti₅O₁₂/n-Ag composites^78,79 and gives insight on the influence of conductive particles morphology and surface coverage on electrochemistry of electrode materials. It also further confirms a positive effect of conductive nanoparticles deposition on electrode's material surface on its electrochemical properties for next generation of lithium-ion batteries. Additional studies are currently in progress, involving Li₄Ti₅O₁₂ modified only with copper element and controlling the morphology of metal deposits by using reducing and complexing agents.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by The Polish National Centre of Research and Development through the research grant PBS1/A1/4/2012 and by The Polish National Centre of Science through the research grant UMO-2014/15/B/ST5/02118.

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