Sodiation Of Hard Carbon: How Separating Enthalpy And Entropy Contributions Can Find Transitions Hidden In The Voltage Profile

Abstract

Sodium‐ion batteries (NIBs) utilize cheaper materials than lithium‐ion batteries (LIBs) and can thus be used in larger scale applications. The preferred anode material is hard carbon, because sodium cannot be inserted into graphite. We apply experimental entropy profiling (EP), where the cell temperature is changed under open circuit conditions. EP has been used to characterize LIBs; here, we demonstrate the first application of EP to any NIB material. The voltage versus sodiation fraction curves (voltage profiles) of hard carbon lack clear features, consisting only of a slope and a plateau, making it difficult to clarify the structural features of hard carbon that could optimize cell performance. We find additional features through EP that are masked in the voltage profiles. We fit lattice gas models of hard carbon sodiation to experimental EP and system enthalpy, obtaining: 1. a theoretical maximum capacity, 2. interlayer versus pore filled sodium with state of charge.

Sodiation energetics in pore size controlled hard carbons determined via entropy profiling

“…After checking for representative cycling behaviour (section 2.2) a constant current/constant voltage (CCCV) charging protocol was performed to ensure a consistent starting sodiation state. This stage consisted of galvanostatic charge at 10 mA/g up to 2.5 V, followed by at least 2 hours of polarisation at 2.5 V. This ensured that the hard carbon commenced as close to fully desodiated as possible, in line with protocols previously applied to Li and Na-ion cells 30,[32][33][34][35] .…”

“…A software interface between the data acquisition unit and the battery cycler allowed real time measurement of temperature, current and voltage to the required resolution. Data points were recorded every 1 s. Further detail of the setup, applied to Li-ion half cells [32][33][34] , Na-ion half cells 30 and commercial cells 35,36 , can be found in the cited works.…”

“…The lack of features leads to ambiguity in determining the transition point between these two regions. However, we showed in a previous work that the transition point can be more objectively determined by measuring the entropy during sodium insertion 30 , via probing the response of the cell open circuit voltage (OCV) to temperature variation at various different sodiation states of hard carbons. With additional input from lattice gas modelling, the technique can separate the voltage response from sodium insertion in the disordered carbon interlayers versus the nanopores 30 .…”

“…However, we showed in a previous work that the transition point can be more objectively determined by measuring the entropy during sodium insertion 30 , via probing the response of the cell open circuit voltage (OCV) to temperature variation at various different sodiation states of hard carbons. With additional input from lattice gas modelling, the technique can separate the voltage response from sodium insertion in the disordered carbon interlayers versus the nanopores 30 . It also allows the voltage itself to be separated into energetic (enthalpy) and configurational (entropy) contributions, which helped to clarify mechanistic details of the various sodium insertion processes.…”

Entropy profiling for the diagnosis of NCA/Gr-SiOx Li-ion battery health

“…Cell temperatures were monitored with type-T thermocouples. A Keysight 34972A data acquisition system with multiplexer unit was used for high resolution (22 bit) voltage and temperature measurements, assisting post processing of entropy profile data, as described in previous publications [23,44,35,25,45].…”