Characterising local environments in high energy density Li-ion battery cathodes: a combined NMR and first principles study of LiFe x Co 1-x PO 4
- Department of Chemistry; University of Cambridge; Cambridge, UK
- Centre de RMN à Très Hauts Champs; 69100 Villeurbanne, France
- Laboratory of Green and Renewable Energy Materials; Department of Materials Science and Engineering; South University of Science and Technology of China; Shenzhen, China
- Department of Physics; University of Warwick; Coventry, UK
- Department of Physics; University of Warwick; Coventry, UK; Tehnomeedikum; Tallinn University of Technology
- Department of Chemistry; University of Cambridge; Cambridge, UK; Department of Chemistry; SUNY Stony Brook
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 LiFexCo1-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 LiFexCo1-xPO4 experimental data. Combining both data sets, the FCS for the range of local P environments expected in LiFexCo1-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 LiFexMn1-xPO4, where all 31P shifts lay within the LiMnPO4–LFP range. Although there are 24 distinct local P environments in LiFexCo1-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.
- Research Organization:
- Energy Frontier Research Centers (EFRC) (United States). Northeastern Center for Chemical Energy Storage (NECCES)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- DOE Contract Number:
- SC0001294
- OSTI ID:
- 1387302
- Journal Information:
- Journal of Materials Chemistry. A, Vol. 2, Issue 30; Related Information: NECCES partners with Stony Brook University (lead); Argonne National Laboratory; Binghamton University; Brookhaven National University; University of California, San Diego; University of Cambridge, UK; Lawrence Berkeley National Laboratory; Massachusetts Institute of Technology; University of Michigan; Rutgers University; ISSN 2050-7488
- Publisher:
- Royal Society of Chemistry
- Country of Publication:
- United States
- Language:
- English
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