No abstract

No abstract

Near zero-volt storage is a promising concept to controllably ensure the safety of user-inactive lithium-ion cells during storage or transit. An advantageous method to enable lithium-ion cells to tolerate near zero volt conditions is to manage the amount of reversible lithium ions in a cell such that during near zero volt storage, the potential of each electrode is maintained outside of its damage range. Unlike current commercial methods, this method can be applied to lithium-ion cells in a full range of cell designs (i.e. high power, high energy density) using current state of the art active materials, electrolytes, and current collectors. In the present work, a scalable reversible lithium management technique using a bath cell is demonstrated for the fabrication of a zero-volt tolerant 225 mAh LiNiCoAlO2/MCMB pouch cell. An electrode asymptotic potential (EAP, the potential at which the electrodes asymptote to when the cell is in a near zero volt state) of 2.2 V vs. Li/Li + was achieved in the final pouch cell as a result of the bathcell reversible lithium management step. The constructed pouch cell demonstrated ~100% discharge capacity and voltage retention after 14 days at a near zero volt state maintained by an applied resistor. Thus, the bath-cell reversible lithium management method can successfully fabricate reversible lithium managed lithium-ion cells that are highly tolerant to a prolonged (> 1 day) zero volt state.

Enhancing the stability of lithium ion cathode active materials to over-insertion of lithium can improve overdischarge tolerance in cells that have excess reversible lithium compared to the stoichiometric cathode capacity. In the present work, a solution deposited coating of AlPO 4 on LiCoO 2 , known to stabilize overcharge, is applied to test its effect on the overdischarge (or over-insertion) tolerance of LiCoO 2 . Cathode testing versus lithium metal is performed with constant current charge to 140 mAh/g LiCoO 2 , constant current discharge to 3.0 V vs. Li/Li + and 7 mAh/g LiCoO 2 lithium over-insertion at fixed resistive load. Results show that the AlPO 4 coating maintains >99% of the discharge energy (compared to 96% for the as-received LiCoO 2 ) after 10 cycles. X-ray diffraction analysis of the relative intensity of {003} peak of the R3M space group indicates that after repeated lithium over-insertion, the AlPO 4 coating suppresses irreversible cation exchange between Li and Co ions in the octahedral layers of LiCoO 2 . Thus, AlPO 4 surface coatings can prevent crystal structure changes detrimental to charge/discharge performance in LiCoO 2 caused by over-insertion of lithium ions. Overdischarge of conventional lithium ion cells can lead to reduced cell performance, cell failure, or safety concerns.1,2 The primary mechanism of overdischarge damage in a conventional lithium ion cell is dissolution of the anode copper current collector, which can lead to internal shorting and capacity loss.3-8 Dissolution of the copper current collector occurs during overdischarge because of the high electrochemical potential that the anode experiences in conventional cells (>3.0 V vs. Li/Li + ). 3,6,9,10 Such a high anode potential during overdischarge ultimately results from loss of reversible lithium to the formation of the solid electrolyte interphase (SEI) during the initial cycling of a cell.9,11 Thus, efforts to protect cells from overdischarge have generally focused on prevention of copper dissolution. 3,6,8,[12][13][14] In advanced lithium ion cells, adding reversible lithium (via high loss cathode materials, anode pre-lithiation, etc.) is utilized to improve general cell performance, 15-17 eliminate the need for formation cycling, 18 manage high first cycle irreversible loss in the anode, 19 and prevent dissolution of the anode copper current collector during overdischarge 3 or near zero volt storage. 9,20 The resulting cell may have excess reversible lithium compared to the normal operation cathode insertion capacity. For such a cell, in an overdischarge scenario, the anode potential will not increase to a high enough value (e.g. >3.1 V vs. Li/Li + ) for copper dissolution to occur. Rather, the cathode potential will decrease to less than its normal range, and the cathode can be over-inserted with lithium at a potential less than its normal operating voltage. 9This over-insertion of lithium can lead to degradation of the cathode performance.9 As such, in advanced lithium ion cells with added reversible lithium, d...

Reduction reactions with lithium at >0.3 V vs Li/Li + make germanium prospectively compatible with aluminum current collectors to form a copper-free lithium-ion cell anode that may enable lithium-ion cells that are more tolerant to overdischarge. Targeted 2.0 mAh/cm 2 germanium nanoparticle electrodes with an aluminum foil (Ge−Al electrode) current collector were fabricated and tested versus lithium from 0.3 to 1.5 V vs Li/Li + in 1.0 M LiPF 6 1:1 ethylene carbonate/diethyl carbonate (v/v) electrolyte and demonstrated a peak reversible capacity of 342 mAh/g Ge at a targeted C/10 rate. A targeted 2.0 mAh/cm 2 germanium nanoparticle electrode with a copper current collector (Ge−Cu electrode) was also tested from 0.005 to 1.5 V vs Li/Li + , the results of which are utilized as a representative comparison of germanium tested in a conventional lithium-ion anode potential range. Compared to the Ge−Cu electrode, after 50 cycles at a targeted C/10 rate the Ge−Al electrode showed 90% vs 18% capacity retention. Post-mortem analysis with XPS, SEM/EDS, Raman spectroscopy, and FTIR showed that compared to the Ge−Cu electrode the Ge−Al electrode had (1) less oxygen, fluorine, and carbon deposition, (2) less germanium amorphization, (3) formation of Li x C y and Ge−H, and (4) less Li 2 O formation. After 150 cycles at equivalent targeted rates, the Ge−Al electrode also had significantly less mid-frequency impedance growth than the Ge−Cu electrode. Modeling of 18650 format LiNiCoAlO 2 -cathode lithium-ion cells with a Ge−Al anode predicted an achievable energy density and specific energy of up to 491 Wh/L and 198 Wh/kg. A germanium dissolution potential of 4.2 V vs Li/Li + was assigned based on voltammetry and post-mortem SEM/EDS. Cycling of a Ge−Al electrode with an upper potential cutoff of 4.2 V vs Li/Li + every tenth cycle showed 30% capacity retention after 50 cycles, an increased capacity fade attributed to solid electrolyte interphase instability up to 4.2 V vs Li/Li + based on impedance and post-mortem analysis.