First 18650-format Na-ion cells aging investigation: A degradation mechanism study

Several Hard carbon||Na3V2(PO4)2F3 full-cells in 18650-format are assembled to demonstrate the possible use of SIBs in stationary applications. The cell aging process is investigated in two different conditions: (i) continuous cycling at different current rates, and (ii) storage at different states-of-charge at various temperatures. The obtained results reveal that the cell degradation depends strongly on the temperature, current rates applied in cycling conditions, or state-of-charge of the storage test. Under cycling conditions, the continuous sodiation/desodiation may induce significant mechanical deformation, leading to the detachment of active materials from the current collector. Furthermore, the post-mortem analysis shows that reaction rate and aging process are not homogeneous along the electrode roll. The XRD analysis shows that Na3V2(PO4)2F3 structure is robust; nevertheless, the material cannot recover the initial Na+ content as the cycling progresses, which is the main cause for capacity loss in the positive electrode. The solid-electrolyte interphase present on the hard carbon surface was characterised using XPS. The hard carbon electrode cannot be detected during this study, evidencing the formation of a relatively thick (>5 nm) passivating layer composed of carbonate salts and NaF, which are the main products of electrolyte decomposition.
The authors acknowledge the European Commission for funding this work through the H2020 NAIADES project (LCE10-2014, Contract number 646433). LC, DC, PSC and LHBN thank the RS2E Network for the funding of LHBN's PhD thesis as well as the financial support of Région Nouvelle Aquitaine. LC, DC, PSC, LHBN, CC, and RD acknowledge the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01) for financial support. MRP and AP acknowledge Ministerio de Economía y Competititivad (Spain) for Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). The authors thank E. Garitte (SAFT, France) for her helps to coordinate this project.