Comparative Evaluation of LMR-NCM and NCA Cathode Active Materials in Multilayer Lithium-Ion Pouch Cells: Part II. Rate Capability, Long-Term Stability, and Thermal Behavior

Abstract

A lithium- and manganese-rich layered transition metal oxide-based cathode active material (LMR-NCM) with a reversible capacity of 250 mAh g−1 vs graphite is compared to an established NCA/graphite combination in multilayer lithium-ion pouch cells with a capacity of 5.5 Ah at a 1C discharge rate. The production of the cells, the electrode characterization as well as the formation is described in Part I of this study. In Part II, the two cell types are evaluated for their rate capability and their long-term stability. The specific capacity of the LMR-NCM pouch cells is ≈30% higher in comparison to the NCA pouch cells. However, due to the lower mean discharge voltage of LMR-NCM, the energy density on the cell level is only 11% higher. At higher discharge currents, a pronounced heat generation of the LMR-NCM pouch cells was observed, which is ascribed to the LMR-NCM voltage hysteresis and is only detectable in large-format cells. The cycling stability of the LMR-NCM cells is somewhat inferior due to their faster capacity and voltage fading, likely also related to electrolyte oxidation. This results in a lower energy density on the cell level after 210 cycles compared to the NCA pouch cells.

Implications of the Heat Generation of LMR-NCM on the Thermal Behavior of Large-Format Lithium-Ion Batteries

“…Therefore, the temperature evolution of one LMR-NCM/ graphite and one NCA/graphite pouch cell measured in discharge rate capability tests in our previous work was used for comparison. 24 During this test, the pouch cells were mounted in cell holders and the temperature was measured with a negative temperature coefficient (NTC) temperature sensor on the surface of the pouch cells with a precision of ±1 K (compare Fig. 1 In the thermal validation model, the cell holder was represented by two aluminum blocks with thicknesses of 20 mm and 15 mm, which were placed on the bottom and top of the pouch cell, respectively, as seen in Fig.…”

“…18,19,24 Thereby, the capacity loss is not only related to structural changes of LMR-NCM, but also to electrolyte stability issues, as LMR-NCM is often cycled up to 4.7-4.8 V vs Li/Li + . 18,21,24 Despite the remaining challenges, the high specific capacity combined with the lower material costs render LMR-NCM to a promising CAM for future lithium-ion batteries. 3,20,25 Meister et al 26 classified the energy efficiency of various CAMs and AAMs for a 1C charge/discharge cycle.…”

“…By adding 1 h open circuit voltage (OCV) rest periods, the voltage difference between charge and discharge can be approximately separated into a voltage drop due to overpotentials during cycling and the LMR-NCM OCV hysteresis. 24 The overpotentials are caused by poor charge transfer kinetics and/or a slow solid-state diffusion of LMR-NCM, especially at low states-of-charge (SOC). 17,28,29 While there still is a minimal voltage relaxation after 1 h OCV, an OCV hysteresis remains for LMR-NCM materials even after resting periods of several days.…”

“…1 indicates the approximate share of the OCV hysteresis of LMR-NCM, which is presumably currentindependent, and the electrical work that is lost due to such a voltage hysteresis is most likely dissipated as waste heat. 20,24 So with rising C-rates, the share of the irreversible heat due to overpotentials likely dominates the total heat generation. In Table I, the round-trip energy efficiencies and maximum surface temperatures for the two pouch cell types are stated (taken from Ref.…”

“…Round-trip energy efficiencies of multilayer LMR-NCM/ graphite and NCA/graphite pouch cells with a nominal capacity of 6-7 Ah during a discharge rate capability test at an ambient temperature of 25°C. 24 The temperatures were measured on the surface of the pouch cells within cell holders and the stated maximum surface temperatures T max,surf were reached during the discharge. ) for LMR-NCM and 9.4 mg cm −2 (≡3.3 mAh cm −2 at C/10, based on a nominal capacity of 355 mAh g graphite 1 -…”