Studies On Zirconium-containing Compound Surface Modification Of Lithium-rich Layered Cathode Material

As the most efficiently commercialized energy storage devices, lithium-ion batteries?LIB_s? have been widely used in portable electronic devices such as mobile phones, laptops, digital cameras, and are also expected to be further applied in electric vehicles, hybrid vehicles, military as well as aerospace fields. With the prominent surge of market demand, LIB_s can no longer meet the high efficiency and high energy and other aspects of more pressing demands. In order to further improve the energy density of LIB_s, we need to develop either a high-capacity or a high-voltage cathode material. Most recently, Li-rich oxide materials have received particular attention for their high reversible capacities?³00 m A h g-1?, which is nearly twice as high as those of LiCoO₂ and LiFePO₄ cathodes presently used in commercial LIB_s.However, Li-rich layered cathode materials have several obstacles: 1) Oxygen loss in the initial charge process will reduce the coulombic efficiency and irreversible capacity; 2) Both the ion rearrangement during cycling and the existence of Li₂MnO₃ deteriorate the cycling performance; 3) In addition, when the electrode is charged to 4.8 V, the side reactions on its surface pose challenge on interface stability. At present, many strategies have been put forward to improve the electrochemical performances of cathode material. Among them, surface coating is the most facile method for its convenience, easy to repeat, low cost and prominent modification effect.In this work, Li[Li_0.2Ni_0.17Co_0.07Mn_0.56]O₂ was synthesized by sol-gel method. Afterwords, surface modification is applied to improve its electrochemical performance. The main works are summarized as follows:1. A conductive Li₂ZrO₃ layer is successfully coated on the surface of Li-rich layered cathode Li[Li_0.2Ni_0.17Co_0.07Mn_0.56]O₂ to enhance its electrochemical performances. The crystal structures, electrochemical properties and thermal stabilities of the bare and coated materials are studied by X-ray diffraction?XRD?, field emission scanning electron microscopy?FESEM?, high resolution transmission electron microscopy?HRTEM?, electron diffraction spectroscopy?EDS?, inductively coupled plasma?ICP?, galvanostatic cycling, cyclic voltammetry?CV?, electrochemical impedance spectroscopy?EIS?, Fourier transform infrared spectroscopy?FTIR? and differential scanning calorimetry?DSC?. It has been found that the electrochemical performances of Li-rich cathode material are obviously improved by Li₂ZrO₃ surface modification. Especially, the 1 wt.% Li₂ZrO₃-coated material demonstrates the best cycling performance, with capacity retention of 89% after 50 cycles, much better than that of the pristine one, 64%. Intensive exploration indicates that the improved electrochemical properties can be attributed to the Li₂ZrO₃ surface layer, which not only stabilizes the cathode structure by decreasing the loss of oxygen, but also protects the Li-rich cathode material from side reaction?s? with the electrolyte and thus suppressing the fast growth of solid electrolyte interface?SEI? film on the surface of oxide particles.2. Layered Li[Li_0.2Ni_0.17Co_0.07Mn_0.56]O₂ is successfully synthesized by a sol-gel method and is further coated with ZrF₄?0.5, 1, 2 and 3 wt %? through a simple wet chemical strategy. Physical characterizations indicate that the ZrF₄ nanocoating layers have little impact on cathode structure. Comparison of electrochemical performances demonstrate that 1 wt % ZrF₄ modified electrode exhibits the highest reversible capacity?193 m Ah g-1? and best cycling performance?capacity retention of 89%? after 100 cycles at 0.1 C. EIS analysis reveals that charge transfer resistance grows much slower after surface coating. FTIR results further confirm that the surface ZrF₄ effectively suppresses the fast growth of SEI film. The improved electrochemical properties are thus attributed to the multifunctional ZrF₄ nanocoating layer, which not only suppresses the side reaction?s? and oxygen loss, but also accelerates the lithium ion transportation due to the reduced resistance. Additionally, differential scanning calorimetry?DSC? tests show that the ZrF₄ layer also helps in enhancing the thermal stability.3. Hetero-structured Li-rich cathode materials Li[Li_0.2Ni_0.17Co_0.07Mn_0.56]O₂ in conjunction with different contents of ZCP coating layers were prepared. Physical characterizations indicate that the ZCP nanocoating layers have little impact on cathode structure. Electrochemical characterizations reveal that the 3 wt.% coated sample delivers an initial discharge capacity of 216 m Ah g-1 with a coulombic efficiency of 80%, compared to 202 m Ah g-1 and 71%, respectively, for the bare sample. Particularly, the coated sample demonstrates excellent cycling stability with a capacity retention of 91% within 100 cycles. The improved electrochemical properties are thus attributed to the ZCP surface layer, which not only prevents electrolyte from eroding the Li-rich core and thus suppressing the fast growth of solid electrolyte interface film and charge transfer resistance on the surface of oxide particles, but also increases the thermal safety of the charged electrode.