Characterising local environments in high energy density Li-ion battery cathodes: a combined NMR and first principles study of LiFexCo1−xPO4

Abstract

Olivine-type LiCoPO₄ (LCP) is a high energy density lithium ion battery cathode material due to the high voltage of the Co²⁺/Co³⁺ redox reaction. However, it displays a significantly poorer electrochemical performance than its more widely investigated isostructural analogue LiFePO₄ (LFP). The co-substituted LiFe_xCo_1−xPO₄ olivines combine many of the positive attributes of each end member compound and are promising next-generation cathode materials. Here, the fully lithiated x = 0, 0.25, 0.5, 0.75 and 1 samples are extensively studied using ³¹P solid-state nuclear magnetic resonance (NMR). Practical approaches to broadband excitation and for the resolution of the isotropic resonances are described. First principles hybrid density functional calculations are performed on the Fermi contact shift (FCS) contributions of individual M–O–P pathways in the end members LFP and LCP and compared with the fitted values extracted from the LiFe_xCo_1−xPO₄ experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFe_xCo_1−xPO₄ have been calculated and used to assign the NMR spectra. Due to the additional unpaired electron in d⁶ Fe²⁺ as compared with d⁷ Co²⁺ (both high spin), LFP is expected to have larger Fermi contact shifts than LCP. However, two of the Co–O–P pathways in LCP give rise to noticeably larger shifts and the unexpected appearance of peaks outside the range delimited by the pure LFP and LCP ³¹P shifts. This behaviour contrasts with that observed previously in LiFe_xMn_1−xPO₄, where all ³¹P shifts lay within the LiMnPO₄–LFP range. Although there are 24 distinct local P environments in LiFe_xCo_1−xPO₄, these group into seven resonances in the NMR spectra, due to significant overlap of the isotropic shifts. The local environments that give rise to the largest contributions to the spectral intensity are identified and used to simplify the assignment. This provides a tool for future studies of the electrochemically-cycled samples, which would otherwise be challenging to interpret.