A Study of Model‐Based Protective Fast‐Charging and Associated Degradation in Commercial Smartphone Cells: Insights on Cathode Degradation as a Result of Lithium Depositions on the Anode

Abstract

mainly due to the steep material development progress in the last 20 years and the resulting long cycle life (>1000 cycles), [3,4] which is especially crucial for devices charged at least one to two times a day. [5-7] However, despite its high theoretical capacity of 274 mAh g −1 , LCO only delivers a useable specific capacity of about 140 mAh g −1 at an upper cutoff voltage of 4.2 V (vs Li/Li +). [8-10] Hence, charging to higher cell cutoff potentials of 4.2-4.5 V has been a way to achieve higher specific energy. [11] However, at potentials above 4.5 V (x = 0.28), LCO shows a fast capacity fade, mainly due to instability of the layered structure and deleterious phase transition. [11,12] Cobalt dissolution, [9] oxygen loss, [13] and side reactions with the electrolyte [9] have also been shown to decrease lifetimes when cells are repeatedly charged to high potentials. Mitigating the issues with LCO at high potentials has attracted a lot of research interest and helped to improve the stability of LCO using surface modifications, including coatings, [14-18] and doping with a variety of elemental compositions. [18-20] Today, these efforts have led to LCO having the largest market share in terms of battery use for mobile consumer applications. [2,4] Besides offering high cycling stability for mobile consumer applications, another The ever expanding mobile consumer electronic market has accelerated the need for safe and efficient fast-charging approaches that improve the overall speed of battery charging without hastened deterioration of the battery performance. Herein, the impact of a resource inexpensive, physics-based, electrochemically optimized fast-charging algorithm (charging time < 2 h) for mobile devices is investigated. A critical difference in the amount and morphology of lithium deposits on the anode for cells fast-charged without an optimized algorithm is observed and found to be the main cause of capacity decay. An in-depth study of the LiCoO 2 cathode regions opposite to pronounced lithium deposits on the anode reveals a "mirroring" phenomenon, i.e., a frozen monoclinic phase, and inactivity to relithiation. In operando hard X-ray absorption spectroscopy reveals that degraded spots on harvested cathodes seem to be activated again and participate in the intercalation process when lithiated at low rates from lithium foil counter electrodes. On the other hand, tests at higher Crates , closer to the actual fast-charging rate, reveal only negligible oxidation state changes and therefore poor performance.

Location-Dependent Cobalt Deposition in Smartphone Cells upon Long-Term Fast-Charging Visualized by Synchrotron X-ray Fluorescence

“…Postmortem analysis of all three cell types, after cycling with their respective OEM cyc protocol, reveals a tremendous difference between the amounts of Li deposited on the anode on a macroscopic scale. While cell types A and B only had minimal Li depositions, primarily around the folds and edges of the negative electrode jelly roll, almost all of the anode in cell type C was covered with Li depositions and electrolyte decomposition products (Figure , also see our previous work analyzing in detail the nature of the anode deposits for this cell type) . At the same time, cell type C showed almost no Li depositions or any Li plating when cycled with the MOD cyc protocol.…”

“…As the cell ages, the optimal fast-charging profile is adjusted to the cell’s state of health (SOH) using proprietary adaptation rules and custom building blocks for charging profile generation . The building blocks for approximation of the new fast-charging profile include, among others, constant current (CC), ramp current, constant voltage (CV), and ramp voltage, while adaptation criteria could be based on state of charge (SOC), SOH, current, voltage, temperature, capacity, and so forth. , …”

“…Our main focus is to shed light on how optimized fast-charging algorithms, proven to be effective for reducing the amount of Li plating on the anode, , can influence the amount of transition-metal dissolution and how much of an influence the anode location within the cell has on the deposition of Co. For this, we used ex situ synchrotron μ-XRF spectroscopy to probe harvested anode samples after two fundamentally different fast-charging protocols: an optimized fast-charging protocol derived using a physics-based electrochemical model and a fast-charging protocol suggested by the original equipment manufacturer (OEM) of the smartphone. Previously, we reported the benefits of using an optimized charging profile and how Li plating can be prevented . Our demonstration here focuses on the protocol-induced differences in the amount and location of deposited Co on the anode and the differences in Li depositions on specific, spatially distinct locations such as edges, folds, and center parts of the negative electrode.…”

“…8,13,14 The application of a model-based, optimized fastcharging profile can help mitigate these types of degradation while providing consumers the freedom to charge their portable electronics in a short amount of time. While it has been shown previously that optimized fast-charging algorithms can enhance battery life cycle as well as reduce Li plating, 12,[15][16][17][18] little information is available on the impact of fast-charging protocols on transition metal dissolution. Several authors have investigated the general concept and effects of transition metal dissolution from layered cathode materials, 8,[19][20][21][22][23] indicating how dissolution of transition metals from layered cathode materials can lead to capacity fade.…”

“…Furthermore, the interplay of these effects can also introduce other phenomena such as transition-metal dissolution of the cathode, especially at higher cutoff voltages, as it is typically believed to be related to electrolyte oxidation and subsequent generation of acidic species in the electrolyte that attack the cathode active material. ,, The application of a model-based, optimized fast-charging profile can help mitigate these types of degradation while providing consumers the freedom to charge their portable electronics in a short amount of time. While it has been shown previously that optimized fast-charging algorithms can enhance battery life cycle as well as reduce Li plating, ,− little information is available on the impact of fast-charging protocols on transition-metal dissolution. Several authors have investigated the general concept and effects of transition-metal dissolution from layered cathode materials, ,,− indicating how dissolution of transition metals from layered cathode materials can lead to capacity fade.…”