V) than LMFP60 (3.64 V), which, when combined with the excess capacity, provided a boost to gravimetric energy density of 23 % (Figure ). Given the higher true density of NMC (4.7 g cm -3 ) vs. LMFP60 (3.6 g cm -3 ), the advantage to volumetric energy density was even more significant at 39 % (Figure ). The inclusion of some NMC also alters the shape of the voltage profile to a more gradual evolution with SOC, beneficial for the determination of state-of-charge (SOC) based on OCV measurement (compared with LMFP alone). The cells were subjected to repeated cycling to assess material stability (Figure ). Both NMC and the blend material maintained an energy density advantage over LMFP throughout 100 cycles. However, LMFP exhibited the greatest stability, maintaining 93 % of initial energy density, while NMC811 fell 85.3 % of initial. Here the NMC811 was cycled to an upper cutoff voltage of 4.4 V, which, while providing high initial performance, induces oxygen loss and the formation of a reduced surface layer. The blend material (also cycled to 4.4 V) exhibited a capacity loss between that of the unblended variants, maintaining 87.7 % of initial energy after 100 cycles. Notably, during the first 55 cycles, the stability trend of the blend material followed that of NMC811 (Figure ) but deviated significantly thereafter (absolute capacity and coulombic efficiency are shown in Figure ). Degradation mechanisms can be further diagnosed via analysis of the evolution of discharge voltage profiles during cycling (Figure ). Initially, the C/3 profiles look similar to those measured at C/20. However, at the 100 th cycle, noticeable changes in profile shape are visible. Firstly, the profile of the NMC811 cell has been depressed due to polarization, attributed to the buildup of kinetically limiting reduced surface layer on the NMC particles. This is consistent with obtained EIS spectra (Figure ), whereby the impedance of the pure NMC and blend cathodes had evolved and increased significantly after 100 cycles, but that of the LMFP60 cell had fallen slightly.