Spatially and Time-Resolved Investigation of Lithium Plating on a Graphite Electrode during Fast Charging Using Operando Neutron Depth Profiling (NDP)

Abstract

Apart from increasing the battery size of electric vehicles (EVs), another approach to reduce range anxiety and to achieve mass market penetration is to increase the charging power and thus significantly lower charging times. To date, commercial lithium-ion battery (LIB) cells are almost exclusively based on graphite as anode material.[1] Here, owing to the low potential of the graphite electrode during charge, the charging rate must be limited in order to avoid lithium plating, which can thermodynamically occur in regions of the electrode where the potential drops below 0 V vs. Li+/Li. Plated lithium is very reactive, leading to irreversible reactions with the electrolyte and therefore strongly reduces lifetime and safety of a battery.[2] Studying lithium plating in a battery cell is very challenging since post mortem diagnostics do not reflect the electrode state during operation, since plated lithium will reintercalate into the active graphite material on a timescale of minutes at room temperature.[3] , [4] Hence, non-destructive operando techniques[5] , [6] are necessary to elucidate and follow lithium plating mechanisms in an operating cell.In this study, we present data collected via Neutron Depth Profiling (NDP) during lithium plating induced by fast charging of a graphite electrode at a rate of 2 C. NDP is a lithium-sensitive, non-destructive technique utilizing the neutron capture reaction of 6Li atoms to quantitatively measure the lithium distribution over the thickness of an electrode by probing the energy loss of charged particles formed by the neutron capture reaction.[7] We employ a recently developed operando NDP cell that shows a cycling performance comparable to a commercial cell even at high charging rates, and at the same time allows charged particles to exit the cell through a thin Kapton® window.[8] We were able to resolve the lithium depth distribution changes during the open circuit voltage (OCV) phase after a constant current charge with a time resolution of 2 min. This exceptionally high time resolution was achieved by combining the high cold neutron flux of 2.7×109 ncm-2s-1 at the NIST Center for Neutron Research (Washington D.C., USA) with a 6Li enriched electrolyte and a 6LiFePO4 cathode, which enhances the signal by an additional factor of 12. In order to achieve lithium plating already at a rate of 2 C, the LiPF6 salt concentration in the EC/EMC = 3/7 (w/w) electrolyte was reduced to 0.3 M.Figure 1 shows the NDP signal intensity as a function of the energy of the detected tritium particles, which translates into a depth profile: signals at an energy of ca. 2400 keV represent lithium at the interface between the graphite electrode and the Cu current collector, and signals at decreasing energies represent lithium from increasing depth into the graphite electrode, with signals at an energy of ca. 800 keV deriving from the graphite anode / separator interface. It is clearly visible that lithium plating starts towards the separator facing side of the graphite anode, caused by the lithium-ion concentration gradient forming over the thickness of the electrode during fast charging (dark blue profile); during the subsequent OCV phase, lithium re-distributes over the anode thickness, whereby this equilibration process is observed to be completed within ca. 24 min.