Operando Analysis of Transition Metal Dissolution from Lithium-Ion Battery Cathodes

The development and commercialisation of rechargeable Li-ion batteries in the 1990s has started a revolutionary shift in consumer electronics and is today the indispensable power source for phones, laptops and other portable devices. This is due to the batteries’ high specific energy density in the range of some hundred Wh/kg (Blomgren, Sagadevan), a consequence of light weight and very low standard potential of metallic lithium, which partly translates to its ions as well; and exceptional cycling stability and efficiency, which enables a long battery lifetime without significant degradation in performance. Depending on cell chemistry and application requirements, i.e. power or energy demand, some thousand charge and discharge cycles or 10 year lifetime (defined as capacity retention over 80% of initial value) are possible. In recent years, the interest in Li-ion batteries has grown even more, as an instrumental part in transport electrification and potential grid storage for renewable energies. Sitting at a produced energy of ca. 700 GWh in 2022, the global lithium ion batteries market may well grow up to 4700 GWh in 2030 according to business analysts (McKinsey), signifying a 30% increase per year. The comprehensive impact of Li-ion battery technology has been recognized with the Nobel Prize in chemistry for Whittingham, Goodenough and Yoshino in 2019. However, as electric vehicles and storage systems require increased power and energy density, respectively, pushing the limits of Li-ion chemistry has become very difficult. The obvious measure, extending the voltage window of the battery and thus increase utilization of Li-ions, also accelerates degradation reactions and shortens the batteries’ lifetime considerably. A major parasitic process in this context is the dissolution of transition metals from the cathode. In the usual potential window of Li-ion batteries, i.e. 3 – 4.2 V vs Li, delithiation and lithiation of the cathode host structure during charge and discharge is highly reversible and efficient, except for Li loss during the first few cycles due to the formation of electrode interface layers. The repeated extraction and insertion of Li-ions is accompanied by changes in the crystal volume – slow expansion of the oxide layers during charge and sudden contraction approaching end of charge - which result in mechanical stress and grinding over extended periods of time. Increasing the cutoff potential during charge severely enhances the structural rearrangements and facilitates TM dissolution. This process coincides with phase transitions of the cathode material, oxygen release and electrolyte decomposition, which in themselves are detrimental for the battery performance. Dissolved transition metals, even worse, may migrate to the anode and deposit into the anode electrolyte interface. The insertion reaction not only increases the thickness of the SEI, which raises the internal resistance, but also traps active Li ions, which reduces the Li inventory. Overall, capacity and capacity retention of the battery deteriorate. Though state of the art cathode materials such as NCM 532 already suffer from transition metal dissolution, potential future materials such as high voltage spinel LMO or LNMO are even more prone to dissolution. Mn ions, which would be beneficial as main metal due to natural abundance, non-toxicity and environmental friendliness, is particularly detrimental to the battery performance upon dissolution, thus preventing the application of corresponding materials. An essential role in the development of next generation Li-ion batteries and counter-measures for TM dissolution is therefore the understanding of fundamental dissolution processes. Although a variety of experimental techniques have been presented in literature, most investigations are performed post-mortem on disassembled cells. There are many reports on operando in-situ studies, which like the ex-situ analyses have given invaluable insight into the dissolution mechanism. However, due to lack of sensitivity or the time frame of most spectroscopic techniques, real time data are scarce. Online analysis of TM dissolution, by monitoring the concentration of TM metals in the electrolyte, can therefore add new insights concerning the dissolution process. In this work, a flow cell coupled to ICP-MS is used to ascertain the change in TM concentration operando at a ppb scale and with a resolution of one second in an intuitive manner. This method can therefore be useful in swiftly elucidating dissolution mechanisms and evaluate influences of operating parameters and materials on TM dissolution.