Electrolyte solution for the improved cycling performance of LiCoPO4/C composite cathodes

The origin of stability and high Co2+/3+ redox utilization for FePO4-coated LiCo0.90Ti0.05PO4/MWCNT nanocomposites for 5 V class lithium ion batteries

Highly-dispersed 10 wt% FePO₄ (FP)-coated LiCo_0.90Ti_0.05PO₄ (LCTP) was successfully synthesized within a multiwalled carbon nanotube matrix via our original ultracentrifugation process. 10 wt% FP-coated LCTP sample showed a higher discharge capacity of 116 mA h g⁻¹ together with stable cycle performance over 99% of capacity retention at the 100^th cycle in high voltage. A combination of TEM, XRD, XPS, and XAFS analyses suggests that (i) Ti⁴⁺-substitution increases the utilization of Co redox (capacity increase) in LCP crystals by suppressing the Co₃O₄ formation and creating the vacancies in Co sites, and (ii) the FP-coating brought about the Fe enrichment of the surface of LCTP which prevents an irreversible crystal structure change and electrolyte decomposition during cycling, resulting in the stable cycle performance.

The Fe³⁺-rich surface on LiCoPO₄ prevents from irreversible crystal structure change and electrolyte decomposition, leading to long term cyclability, while Ti⁴⁺-substitution contributes to the higher utilization of Co in LiCo_0.9Ti_0.05PO₄ crystals.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Lithium cobalt phosphate electrode for the simultaneous determination of ascorbic acid, dopamine, and serum uric acid by differential pulse voltammetry

Lithium cobalt phosphate (LCP) was prepared and modified on the surface of glassy carbon electrode (GCE) to fabricate the electrochemical sensor (LCP/GCE) for the simultaneous determination of ascorbic acid (AA), dopamine (DA), and serum uric acid (UA). The homogenous incorporation of carbon improved the conductivity of LCP. Benefiting from the small particle size distribution, LCP/GCE has a large active surface and responds to AA, DA, and UA sensitively and rapidly. For the simultaneous detection with differential pulse voltammetry the anodic peaks of AA, DA, and UA were well-separated and appeared at ~0 V, ~0.19 V, and ~ 0.33 V (vs. Ag/AgCl), respectively. The linear responses toward AA, DA, and UA were in the range 10 μM-8.0 mM, 10 nM-10 μM, and 0.020 μM-25 μM; the detection limits were estimated to be 8.10 μM, 7.50 nM, and 22.7 nM (S/N = 3), respectively. The excellent selectivity and reproducibility of LCP/GCE enable serum UA to be detected without the interference of AA and DA. The recoveries of DA and AA in the serum sample were in the range 95 to 111%. The results indicate that LCP has the potential to be developed as the sensing devices to be applied to in vitro diagnosis. The lithium-ion battery cathodic material, LCP with the excellent adsorption and catalytic behavior, was utilized to fabricate the electrochemical sensor for the sensitive and simultaneous detection of AA, DA, and UA, which achieved the low detection limits and the wide concentration ranges. LCP/GCE can be used for the quantitative detection of serum UA without the interference of DA and AA. In addition, the recoveries of DA and AA in human serum were satisfactory, which illustrate the reliability of LCP/GCE to be applied to in vitro diagnosis.

Constructing a Low-Impedance Interface on a High-Voltage LiNi0.8Co0.1Mn0.1O2 Cathode with 2,4,6-Triphenyl Boroxine as a Film-Forming Electrolyte Additive for Li-Ion Batteries

Compared with other commercial cathode materials, the LiNi_0.8Co_0.1Mn_0.1O₂ cathode (NCM811) has high specific capacity and a relatively low cost. Nevertheless, the higher nickel content in NCM811 leads to an extremely unstable interface between the electrode and the electrolyte, resulting in inferior cyclic stability of the corresponding cell. Use of film-forming additives is regarded as the most feasible and economic approach to construct a stable interface on the NCM811 cathode. However, less effective electrolyte additives have been reported to date. Herein, we propose a valid film-forming electrolyte additive, 2,4,6-triphenyl boroxine (TPBX), for application in a high-voltage NCM811 cathode. Experimental and computational results reveal that the TPBX additive can be preferentially oxidized to generate a highly stable and conductive cathode electrolyte interface (CEI) layer on the NCM811 cathode, which efficiently suppresses the detrimental side reaction and improves the electrochemical performance eventually. In detail, the cyclic stability of the Li/NCM811 half-cell is enhanced from 57% (without additive) to 78% (with 5% TPBX) after 200 cycles at 1C between 3.0 and 4.35 V. At a high current rate of 15C, the TPBX-containing electrode delivers a capacity of about 135 mAh g^-1, which is much higher than that of the electrode without the additive (80 mAh g^-1). Interestingly, the TPBX is also reduced earlier than the ethylene carbonate (EC) solvent to form an ionically conductive solid electrolyte interface (SEI) film on the graphite anode. Due to the CEI layer on the cathode and the SEI film on the anode simultaneously formed by the TPBX additive, the cyclic performance of the graphite/LiNi_0.8Co_0.1Mn_0.1O₂ full cell is enhanced. Therefore, the incorporation of the TPBX additive into the electrolyte provides a convenient method for the commercial application of the high-energy-density NCM811 cathode in high-voltage lithium-ion batteries.

Trimethyl Borate as Film-Forming Electrolyte Additive To Improve High-Voltage Performances

Enhancing the stability of the interface between the electrode and electrolyte at high voltages is crucial concerning the development of Li-ion batteries with high energy densities. Application of some additives in the electrolyte is not only the simplest but also the most effective way to form a protection layer against the electrolyte decomposition and the electrolyte corrosion to the electrode. Herein, we introduce trimethyl borate (TMB) as an additive of the commercial electrolyte to ameliorate the performance of a LiCoO₂ cell charged to 4.5 V because its addition lowers the oxidation potential of the baseline electrolyte (3.75 V vs 4.25 V). By being oxidized preferentially and thus forming a compact protection layer of about 25 nm thick on the cathode surface, the additive suppresses the electrolyte decomposition and protects the LiCoO₂ cathode against the structural degradation. The capacity retention of the cell after 100 cycles between 2.5 and 4.5 V at 0.1 C increases from 64 to 81% when 2.0 wt % TMB is added into the baseline electrolyte. The X-ray photoelectron spectroscopic results demonstrate the oxidation of TMB on the cathode and therefore the suppressed decomposition of the electrolyte. The results of the X-ray diffraction and Raman spectroscopy show the better structural maintenance of the LiCoO₂ material in the TMB-containing electrolyte. The protection mechanism of the TMB additive was comprehensively studied.

Non-flammable electrolyte enables Li-metal batteries with aggressive cathode chemistries

Rechargeable Li-metal batteries using high-voltage cathodes can deliver the highest possible energy densities among all electrochemistries. However, the notorious reactivity of metallic lithium as well as the catalytic nature of high-voltage cathode materials largely prevents their practical application. Here, we report a non-flammable fluorinated electrolyte that supports the most aggressive and high-voltage cathodes in a Li-metal battery. Our battery shows high cycling stability, as evidenced by the efficiencies for Li-metal plating/stripping (99.2%) for a 5 V cathode LiCoPO₄ (~99.81%) and a Ni-rich LiNi_0.8Mn_0.1Co_0.1O₂ cathode (~99.93%). At a loading of 2.0 mAh cm^-2, our full cells retain ~93% of their original capacities after 1,000 cycles. Surface analyses and quantum chemistry calculations show that stabilization of these aggressive chemistries at extreme potentials is due to the formation of a several-nanometre-thick fluorinated interphase.

A High Voltage Olivine Cathode for Application in Lithium-Ion Batteries

A new olivine composition (i.e., LiFe0.25 Mn0.5 Co0.25 PO4) is proposed as electrode material with increased energy density for application in lithium-ion batteries. The new formulation increases the working voltage and induces different electrochemical behavior with respect to bare olivine materials based on Fe. The study provides deep insight into the features of the Fe(3+) /Fe(2+), Mn(3+)/Mn(2+), and Co(3+)/Co(2+) redox couples within the olivine lattice in terms of electrochemical activity, Li(+) transport properties, and Li-cell behavior. The electrochemical characterization clearly reveals the voltage signatures corresponding to the various metals; however, the Mn(3+)/Mn(2+) process has higher intrinsic polarization with respect to Fe(3+)/Fe(2+) and Co(3+)/Co(2+). This issue is efficiently mitigated by carbon coating the material, resulting in enhanced electrochemical performances.

Facile, ethylene glycol-promoted microwave-assisted solvothermal synthesis of high-performance LiCoPO4 as a high-voltage cathode material for lithium-ion batteries

A simple and rapid microwave-assisted solvothermal synthesis delivers hexagonal platelets of LiCoPO₄ with tuned crystal orientations and leading-edge electrochemical properties.

New Horizons for Conventional Lithium Ion Battery Technology

Secondary lithium ion battery technology has made deliberate, incremental improvements over the past four decades, providing sufficient energy densities to sustain a significant mobile electronic device industry. Because current battery systems provide ∼100-150 km of driving distance per charge, ∼5-fold improvements are required to fully compete with internal combustion engines that provide >500 km range per tank. Despite expected improvements, the authors believe that lithium ion batteries are unlikely to replace combustion engines in fully electric vehicles. However, high fidelity and safe Li ion batteries can be used in full EVs plus range extenders (e.g., metal air batteries, generators with ICE or gas turbines). This perspective article describes advanced materials and directions that can take this technology further in terms of energy density, and aims at delineating realistic horizons for the next generations of Li ion batteries. This article concentrates on Li intercalation and Li alloying electrodes, relevant to the term Li ion batteries.

Fluoroethylene carbonate as an important component in electrolyte solutions for high-voltage lithium batteries: role of surface chemistry on the cathode

The effect of fluorinated ethylene carbonate (FEC) as a cosolvent in alkyl carbonates/LiPF6 on the cycling performance of high-voltage (5 V) cathodes for Li-ion batteries was investigated using electrochemical tools, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HRSEM). An excellent cycling stability of LiCoPO4/Li, LiNi0.5Mn1.5O4/Si, and LiCoPO4/Si cells and a reasonable cycling of LiCoPO4/Si cells was achieved by replacing the commonly used cosolvent ethylene carbonate (EC) by FEC in electrolyte solutions for high-voltage Li-ion batteries. The roles of FEC in the improvement of the cycling performance of high-voltage Li-ion cells and of surface chemistry on the cathode are discussed.