Dong Luo

One-Step Calcination-Free Synthesis of Multicomponent Spinel Assembled Microspheres for High-Performance Anodes of Li-Ion Batteries: A Case Study of MnCo2O4

Multicomponent spinel metal-oxide assembled mesoporous microspheres, promising anode materials for Li-ion batteries with superior electrochemical performance, are usually obtained using different kinds of precursors followed by high-temperature post-treatments. Nevertheless, high-temperature calcinations often cause primary particles to aggregate and coarsen, which may damage the assembled microsphere architectures, leading to deterioration of electrochemical performance. In this work, binary spinel metal-oxide assembled mesoporous microspheres MnCo2O4 were fabricated by one-step low-temperature solvothermal method through handily utilizing the redox reaction of nitrate and ethanol. This preparation method is calcination-free, and the resulting MnCo2O4 microspheres were surprisingly assembled by nanoparticles and nanosheets. Two kinds of MnCo2O4 crystal nucleus with different exposed facet of (1̅10) and (11̅2̅) could be responsible for the formation of particle-assembled and sheet-assembled microspheres, respectively. Profiting from the self-assembly structure with mesoporous features, MnCo2O4 microspheres delivered a high reversible capacity up to 722 mAh/g after 25 cycles at a current density of 200 mA/g and capacities up to 553 and 320 mAh/g after 200 cycles at a higher current density of 400 and 900 mA/g, respectively. Even at an extremely high current density of 2700 mA/g, the electrode still delivered a capacity of 403 mAh/g after cycling with the stepwise increase of current densities. The preparation method reported herein may provide hints for obtaining various advanced multicomponent spinel metal-oxide assembled microspheres such as CoMn2O4, ZnMn2O4, ZnCo2O4, and so on, for high-performance energy storage and conversion devices.

K+-Doped Li1.2Mn0.54Co0.13Ni0.13O2: A Novel Cathode Material with an Enhanced Cycling Stability for Lithium-Ion Batteries

Li-rich layered oxides have attracted much attention for their potential application as cathode materials in lithium ion batteries, but still suffer from inferior cycling stability and fast voltage decay during cycling. How to eliminate the detrimental spinel growth is highly challenging in this regard. Herein, in situ K(+)-doped Li1.20Mn0.54Co0.13Ni0.13O2 was successfully prepared using a potassium containing α-MnO2 as the starting material. A systematic investigation demonstrates for the first time, that the in situ potassium doping stabilizes the host layered structure by prohibiting the formation of spinel structure during cycling. This is likely due to the fact that potassium ions in the lithium layer could weaken the formation of trivacancies in lithium layer and Mn migration to form spinel structure, and that the large ionic radius of potassium could possibly aggravate steric hindrance for spinel growth. Consequently, the obtained oxides exhibited a superior cycling stability with 85% of initial capacity (315 mA h g(-1)) even after 110 cycles. The results reported in this work are fundamentally important, which could provide a vital hint for inhibiting the undesired layered-spinel intergrowth with alkali ion doping and might be extended to other classes of layered oxides for excellent cycling performance.

Gel-combustion synthesis of Li1.2Mn0.4Co0.4O2 composites with a high capacity and superior rate capability for lithium-ion batteries

Owing to the merits of high capacity and low cost, Li-rich layered composite cathode materials have received extensive attention. Nevertheless, such materials always suffer from a poor rate capability, which seriously hinders their widespread practical applications. In this work, Li1.2Mn0.4Co0.4O2 composites were fabricated by a gel-combustion method, in which lithium carbonates formed by an in situ burning reaction were homogeneously mixed with metal oxides, leading to excellent electrochemical properties. The sintering temperature and time were optimized to 900 °C and 15 h. The samples prepared at optimum conditions exhibited a high discharge capacity and excellent rate capability. At a current density of 20 mA g−1, the specific discharge capacity was 310.5 mA h g−1 for the first cycle and the capacity retention was 75.8% after 30 cycles. When the current densities increase by 10 times to reach 200 mA g−1, the initial discharge capacity is still as high as 241.4 mA h g−1, superior to that of 203 mA h g−1 at the same current density reported previously by the oxalate-precursor method. Even when the current densities increase by 20 times, the capacity remained as high as 203.7 mA h g−1, and the capacity retention was 79.4% over 30 cycles. The high discharge capacity and improved rate capability of the optimized sample were beneficial with a perfect layered structure with c/a >5.0, appropriate particle size of about 450 nm, high lithium ion diffusion coefficient around 1.42 × 10−13 cm2 s−1, and the presence of a maximum content of Mn3+ (11.6%), as determined by XPS. The preparation method reported herein may provide hints for obtaining various advanced Li-rich layered composite materials for use in high-performance energy storage and conversion devices.

Nickel-Rich Layered Microspheres Cathodes: Lithium/Nickel Disordering and Electrochemical Performance

Nickel-rich layered metal oxide materials are prospective cathode materials for lithium ion batteries due to the relatively higher capacity and lower cost than LiCoO2. Nevertheless, the disordered arrangement of Li(+)/Ni(2+) in local regions of these materials and its impact on electrochemistry performance are not well understood, especially for LiNi(1-x-y)Co(x)Mn(y)O2 (1-x-y > 0.5) cathodes, which challenge ones ability in finding more superior cathode materials for advanced lithium-ion batteries. In this work, Ni-Co-Mn-based spherical precursors were first obtained by a solvothermal method through handily utilizing the redox reaction of nitrate and ethanol. Subsequent sintering of the precursors with given amount of lithium source (Li-excess of 5, 10, and 15 mol %) yields LiNi0.7Co0.15Mn0.15O2 microspheres with different extents of Li(+)/Ni(2+) disordering. With the determination of the amounts of Li(+) ions in transition metal layer and Ni(2+) ions in Li layer using structural refinement, the impact of Li(+)/Ni(2+) ions disordering on the crystal structure, valence state of nickel ions, and electrochemical performance were investigated in detailed. It is clearly demonstrated that with increasing the amount of lithium source, lattice parameters (a and c) and interslab space thickness of unit cell decrease, and more Li(+) ions incorporated into the 3a site of transition metal layer which leads to an increase of Ni(3+) content in LiNi0.7Co0.15Mn0.15O2 as confirmed by X-ray photoelectron spectroscopy and a redox titration. Moreover, the electrochemical performance for as-prepared LiNi0.7Co0.15Mn0.15O2 microspheres exhibited a trend of deterioration due to the changes of crystal structure from Li(+)/Ni(2+) mixing. The preparation method and the impacts of Li(+)/Ni(2+) ions disordering reported herein for the nickel-rich layered LiNi0.7Co0.15Mn0.15O2 microspheres may provide hints for obtaining a broad class of nickel-rich layered metal oxide microspheres with superior electrochemical performance.

Self-adjusted oxygen-partial-pressure approach to the improved electrochemical performance of electrode Li[Li0.14Mn0.47Ni0.25Co0.14]O2 for lithium-ion batteries

We initiated a self-adjusted oxygen-partial-pressure approach to prepare high-performance Li2MnO3–LiMO2 cathode material. Four different lithium resources, lithium acetate, lithium hydrate, lithium carbonate, and lithium nitrate were used to create the local oxygen partial pressure over the samples. Since the melting points or decomposition temperatures for these lithium resources decrease in a sequence, Li2CO3 ≈ LiOH > LiNO3 > CH3COOLi, the oxygen partial pressure of the four crucibles that contain these lithium salts increases in a sequence, S4 ≈ S3 < S2 < S1 ≈ air in muffle furnace (S1: CH3COOLi·2H2O, S2: LiNO3, S3: LiOH·H2O, and S4: Li2CO3). Regardless of the lithium resources, the decomposed gases reduced the local oxygen partial pressures, leading to an incomplete oxidation of Mn ions in the final product Li[Li0.14Mn0.47Ni0.25Co0.14]O2. That is, some of the Mn3+ ions existed in the final product Li[Li0.14Mn0.47Ni0.25Co0.14]O2, and the amount of Mn3+ ions was closely related to the oxygen partial pressure. The lower oxygen partial pressure gave rise to a larger amount of Mn3+ in the final products, as confirmed by X-ray photoelectron spectroscopy. Electrochemical tests showed that the products prepared using lithium carbonate exhibited the best electrochemical performance: the initial discharge capacity was 279.4 mA h g−1 at a current density of 20 mA g−1, which remained as high as 187.2 mA h g−1 even at a much higher current density of 500 mA g−1. Such excellent electrochemical performance could be ascribed to the presence of Mn3+ that decreased the surface layer resistance and charge transfer resistance, and that further increased the conductivity and Li+ ion diffusion coefficient.

Novel synthesis of Li1.2Mn0.4Co0.4O2 with an excellent electrochemical performance from −10.4 to 45.4 °C

Lithium-ion batteries continue to dominate the market and transportation applications of portable electronics, while these applications are still very difficult at low or elevated temperatures. In this work, the cathode material Li1.2Mn0.4Co0.4O2 was initially synthesized via an oxalate-precursor method. During sample preparation, lithium ions were co-precipitated with transition metal ions to form a uniform distribution of reactants at the molecular level. As a consequence, the current preparation method gave rise to a uniform cation distribution inside the target materials with no need of the additional process of mixing with lithium salt, which is however always required when using conventional co-precipitation methods. Due to the uniform cation distribution inside the material, the Li1.2Mn0.4Co0.4O2 cathode thus prepared was found to exhibit an excellent electrochemical performance. At room temperature, the initial discharge capacity and capacity retention ratio after 20 cycles were 284 mA h g−1 and 82.75%, respectively, which are superior to 246 mA h g−1 and 79.27%, the best results ever reported for the counterparts. Further, the low and elevated-temperature electrochemical performance for this cathode was also explored. It was found that the maximal discharge capacity measured at a current density of 20 mA g−1 between 2.0 and 4.6 V was maintained as high as 296 and 200 mA h g−1, at 45.4 and −10.4 °C, respectively. The change in the state of health (SOH) in the temperature range −10.4 to 45.4 °C was investigated by EIS. It was demonstrated that the %SOH operation window for Li1.2Mn0.4Co0.4O2/Li cells was somewhat broad, which indicated a potential application at low/elevated temperatures. The synthetic route described in this work is new, and may help to prepare more advanced cathode materials essential for a broad class of applications.

Low-concentration donor-doped LiCoO2 as a high performance cathode material for Li-ion batteries to operate between −10.4 and 45.4 °C

The majority of electrode materials suffer from severe capacity fading on cycling at elevated temperatures or poor conductivity and diffusion of Li+ at low temperatures, which have made it very difficult for lithium-ion batteries to operate at low temperatures and/or elevated temperatures without loss of electrochemical performance. In this work, we report on a new strategy for tackling this issue through low-concentration donor-doping of higher valence Mn ions in LiCoO2, a typical commercial cathode for many lithium-ion batteries. Firstly, low-concentration Mn-doped LiCoO2 was successfully synthesized using a molten-salt method, in which solvent NaOH provides an alkaline environment that makes the reactant mixture uniform in reaction process and ensures the valence state of Mn ion at +4. Secondly, the chemical compositions for all samples were systematically tuned, while retaining the single-phase nature. The electrochemically inert Mn4+ was found to significantly enhance the structure stability, conductivity, and diffusion rate of LiCoO2. As a consequence, the cathode material with a composition of LiCo0.95Mn0.05O2 exhibited an excellent electrochemical performance in a temperature range from −10.4 to 45.4 °C. The finding reported in this work will be conducive to the applications of lithium-ion batteries under different temperature conditions.

Balancing stability and specific energy in Li-rich cathodes for lithium ion batteries: a case study of a novel Li–Mn–Ni–Co oxide

Lithium batteries for UPS, portable electronics and electrical vehicles rely on high-energy cathodes. Li-rich manganese-rich oxide (xLi2MnO3·(1 − x)LiMO2, M = transition metals) is one of the few materials that might meet such a requirement, but it suffers from poor energy retention due to serious voltage and/or capacity fade, which challenges its applications. Here we show that this challenge can be addressed by optimizing the interactions between the components Li2MnO3 and LiMO2 in the Li-rich oxide (i.e. stabilizing the layered structure through Li2MnO3 and controlling Li2MnO3 activation through LiMO2). To realize this synergistic effect, a novel Li2MnO3-stabilized Li1.080Mn0.503Ni0.387Co0.030O2 was designed and prepared using a hierarchical carbonate precursor obtained by a solvo/hydro-thermal method. This layered oxide is demonstrated to have a high working voltage of 3.9 V and large specific energy of 805 W h kg−1 at 29 °C as well as impressive energy retention of 92% over 100 cycles. Even when exposed to 55 °C, energy retention is still as high as 85% at 200 mA g−1. The attractive performance is most likely the consequence of the balanced stability and specific energy in the present material, which is promisingly applicable to other Li-rich oxide systems. This work sheds light on harnessing Li2MnO3 activation and furthermore efficient battery design simply through compositional tuning and temperature regulation.

Direct synthesis of carbon-coated Li4Ti5O12 mesoporous nanoparticles for high-rate lithium-ion batteries

The carbon-coated Li4Ti5O12 mesoporous nanoparticles were directly synthesized by a facile solvothermal method with a subsequent N2 treatment. The resultant Li4Ti5O12 did not show any by-products of TiN or TiC, but a unique mesoporous structure and particle dimension of about 20 nm. Confined to the surface thin layer <5 nm of this mesoporous structure was carbon species, which amounts to 0.98 wt%, much less than that when using conventional carbon-coating method. Systematic electrochemical tests on these mesoporous nanoparticles demonstrated an excellent cycling performance with Coulombic efficiencies being all higher than 99% after the initial cycle. Further, when cycled at a high rate of 10 C, these mesoporous nanoparticles delivered an initial discharge capacity of 137 mAh g−1 and maintained a high reversible capacity of 107 mAh g−1 after 1600 cycles, which is apparently much higher than that of 67 mAh g−1 for the first discharge capacity of non-mesoporous Li4Ti5O12. The improved electrochemical performances of Li4Ti5O12 can be attributed to the mesoporous structure and thin carbon coating on Li4Ti5O12 nanoparticles, which facilitates electron and lithium-ion diffusion.