DESCRIPTION OF THE INVENTION

[0017] In certain embodiments of the invention, capacity retention of a lithium-ion battery electrode material is improved by surface/chemical modification. Surface/Chemical modification of oxide electrode materials is typically performed by using a variety of materials including, but not limited to, Li _x CoO ₂ , Li _x Cu _0.5 Ni _0.5 O ₂ , Li _x Co _0.75 Ni _0.75 O ₂ , Al ₂ O ₃ , MgO, MgAl ₂ O ₄ , and Li _1.05 Mn _1.9 Ni _0.05 O ₄ ; where 0≦x≦1.

[0018] Chemically Modified LiMn ₂ O ₄ Cathodes

[0019] In certain embodiments the surface of LiMn ₂ O ₄ spinel oxide is modified to improve capacity retention. The surface/chemically modified LiMn ₂ O ₄ demonstrates an improved capacity retention as compared to unmodified LiMn ₂ O ₄ spinel oxide both at ambient temperature and at elevated temperatures. In certain embodiments the surface of LiMn ₂ O ₄ spinel oxide is modified with surface modification materials such as Li _x CoO ₂ , Li _x Ni _0.5 Ni _0.5 O ₂ , Al ₂ O ₃ , MgO, and/or MgAl ₂ O ₄ (where 0≦x≦1). Surface/chemical modification protects the spinel particles from attack by the acidic species present in the electrolyte and leads to maintenance of good structural integrity during cycling. The surface/chemically modified LiMn ₂ O ₄ spinel oxides may in fact offer better long-term cyclability characteristics and safety features than the commercially used LiCoO ₂ cathodes. The lower cost coupled with high rate capability and excellent cycling properties make the surface/chemically modified LiMn ₂ O ₄ cathodes attractive for energy storage for a variety of uses including, but not limited to cell phones, laptop computers, electric vehicles, and the like.

[0020] In one embodiment the surface/chemical modification of LiMn ₂ O ₄ using a LiNi _0.5 Co _0.5 O ₂ , as described herein, provides superior capacity retention. In another embodiment the surface/chemical modification of LiMn ₂ O ₄ using Li _0.75 CoO ₂ , as described herein, provides a superior combination of capacity value and capacity retention.

[0021] Surface/Chemically-Modified LiCoO ₂ Cathodes

[0022] Typically, commercial lithium ion batteries use the layered LiCoO ₂ oxide cathodes, as they offer better cyclability. However, only 50% of its theoretical capacity could be practically utilized, which corresponds to a reversible extraction/insertion of 0.5 lithium ions per cobalt in LiCoO ₂ . This results in capacity fading above a cut-off charge voltage of approximately greater than 4.2 V. The limited capacity of LiCoO ₂ may be due to its chemical instability and tendency to lose oxygen from lattice on extracting more than 0.5 lithium ions per cobalt.

[0023] In certain embodiments, chemical instability of LiCoO ₂ may be overcome by surface/chemical modification with various compositions. The surface/chemically modified LiCoO ₂ exhibits higher capacity than the unmodified LiCoO ₂ layered oxide cathode both at room temperature and at elevated temperatures with good cyclability. In certain embodiments, the surface of LiCoO ₂ is modified with surface/chemical modification materials such as Al ₂ O ₃ and Li _1.05 Mn _1.9 Ni _0.05 O ₄ . The surface/chemical modification may also improve the safety characteristics of the LiCoO ₂ cathode.

[0024] Electrode Material

[0025] In certain embodiments the electrode material comprises LiMn ₂ O ₄ or LiCoO ₂ . Alternatively, other materials including LiNi _1-y M _y O ₂ where 0≦y≦1 and M=Ti, V, Cr, Mn, Fe, and Cu, LiMn _1-y M _y O ₂ where 0≦y≦1 and M=Cr and Al, and Li _1+x Mn _2-x-y M _y O _4-z+δ X _z where 0≦x≦0.33, 0≦y≦1, 0≦δ≦0.5, M=Mg, Al, Ti, V, Cr, Fe, Co, Ni, Cu and Zn, and X=F and S may also be used as electrode materials.

[0026] Surface/Chemical Modification Materials

[0027] In certain embodiments surface/chemical modification materials may be a ceramic material, such as Li _x CoO ₂ , Li _x Co _0.5 Ni _0.5 O ₂ , Al ₂ O ₃ , Cr ₂ O ₃ , MgO, MgAl ₂ O ₄ , and/or Li _1.05 Mn _1.9 Ni _0.05 O ₄ (where 0≦x≦1) are used to modify the surface of electrode materials. Surface/chemical modification with LiCo _0.5 Ni _0.5 O ₂ may show excellent capacity retention and superior rate capability with a capacity fade of <0.03% per cycle over 100 charge/discharge cycles at 60° C. and 0.5 mA/cm ² (C/2 rate). Other potential surface modification materials include Li _x Ni _1-y M _y O ₂ , where 0≦x≦1, 0≦y≦1, and M=Mg, Al, Ti, V, Cr, Fe, Co, Cu, Zn, and Ga; Al ₂ O ₃ ; MgO; Al _2-y Mg _y O _3-0.5 y where 0≦y≦2; Li _1+x Mn _2-x-y M _y O ₄ where 0≦x≦0.33, 0≦y≦2 and M=Mg, Al, Ti, V, Cr, Fe, Co, Ni, Cu and Zn; Zr _1-y M _y O _2-y where 0≦y≦1 and M=Mg, Ca; Zr _1-y M _y O _2-0.5y where 0≦y≦1 and M=Sc, Y; and combinations thereof

[0028] Methods of Surface/Chemical Modification

[0029] In one embodiment, surface/chemical modified electrode materials are prepared by firing a mixture of electrode material and surface/chemical modifier. Firing temperatures may be in the approximate range of 100° C. to about 1000° C., preferably in the approximate range of 200° C. to 900° C., and also preferably in the approximate range of 300° C. to 800° C. A mixture of electrode material and surface/chemical modifier may be fired for various lengths of time, which may be in the approximate range of 1 to 24 h. Surface/chemical modification of an electrode material may be performed by treating various amounts of an electrode material with various amounts of surface/chemical modification material(s). The process typically results in a product with a surface/chemical modification material content in the range of about 1 weight percent to about 20 weight percent. The preferred surface/chemical modification material content in the approximate range of 2 to 5 weight percent. The process may involve dissolution of carbonates, nitrates or acetates of the surface/chemical modification material(s) in glacial acetic acid, refluxion of the mixture for about an hour, dispersion of an electrode material in a surface/chemical modifier solution, evaporation of the solvent, and decomposition of the resultant product at elevated temperature. The mixture is then fired at an elevated temperature in the presence or absence of a flowing oxygen atmosphere.

[0030] In another embodiment, the surface/chemical modifications with Al ₂ O ₃ , Cr ₂ O ₃ , MgO and MgAl ₂ O ₄ may involve dispersion of an electrode material in an aqueous solution of aluminum, chromium or magnesium nitrate, formation/precipitation of a gelatinous Al(OH) ₃ , Cr(OH) ₃ or Mg(OH) ₂ over source material particles through the addition of ammonium hydroxide, and heating the resultant product at the approximate range of 100° C. to about 1000° C., preferably in the approximate range of 300° C. to 800° C., and also preferably in the approximate range of 300° C. to 400° C. for Al(OH) ₃ and 400° C. to 600° C. for Cr(OH) ₃ or Mg(OH) ₂ .

[0031] Other methods known in the art may be used to modify an electrode material as described herein, including chemical vapor deposition and other similar methods.

[0032] Electrode Fabrication

[0033] Electrodes for use in energy storage and conversion devices, including batteries, may be fabricated by further processing the composites disclosed herein by, for example, grinding to form an electrode. Examples of forming a battery electrode and battery are known to one of ordinary skill in the art. As used herein, “grinding” refers to mixing, crushing, pulverizing, pressing together, polishing, reducing to powder or small fragments, milling, ball milling, or any other suitable process to wear down a material. A conducting material may be mixed with the composites in the process of forming an electrode. The conducting material may be an electrically conductive material such as carbon, which may be in the form of graphite or acetylene black, but it will be understood with benefit of this disclosure that the conducting material may alternatively be any other material or mixtures of suitable materials known in the art.

[0034] Electrodes may be formed in a variety of shapes, sizes, and/or configurations as is known in the art. In one embodiment, electrodes may be formed by rolling a mixture of composites disclosed herein, conducting material, and binding material into one or more thin sheets which may be cut to form, for example, circular electrodes of various surface area, thickness, and weight. Electrochemical performance of such electrodes is typically evaluated according to procedures known in the art.

EXAMPLES

Surface/Chemical Modification of Electrode Material

[0035] The following examples are included to demonstrate various embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques that function in the practice of the invention. However, those of ordinarily skilled in the art may appreciate that many changes can be made in the specific embodiments which are disclosed without departing from the spirit and scope of the invention.

Example 1

Surface/Chemical Modification of LiMn ₂ O ₄ Spinel Oxide

[0036] Material and Methods

[0037] A commercially available LiMn ₂ O ₄ powder may be used as the host electrode material. The surface/chemical modification may be carried out by treating various amounts of LiMn ₂ O ₄ powder with a precursor solution of Li _x CoO ₂ , Li _x Co _0.5 Ni _0.5 O ₂ , Al ₂ O ₃ , MgO, Cr ₂ O ₃ or MgAl ₂ O ₄ through a chemical process so that the amount of the guest modification material in the final product is approximately 3 to 5 wt %. The chemical process in the case of Li _x CoO ₂ , and Li _x Co _0.5 Ni _0.5 O ₂ involve a dissolution of the carbonates or acetates of the precursor metal ions in glacial acetic acid, refluxion of the mixture for about an hour, dispersion of the LiMn ₂ O ₄ spinel oxide in the precursor solution, evaporation of the solvent, and decomposition of the resultant product at around 400° C. The sample is then fired at 850° C. in flowing oxygen atmosphere. The surface/chemical modifications with Al ₂ O ₃ , Cr ₂ O ₃ , MgO and MgAl ₂ O ₄ involve the dispersion of the LiMn ₂ O ₄ spinel oxide in an aqueous solution of aluminum, chromium, or magnesium nitrate or a mixture of aluminum and magnesium nitrates, formation/precipitation of a gelatinous Al(OH) ₃ , Cr(OH) ₃ or Mg(OH) ₂ over LiMn ₂ O ₄ particles through the addition of ammonium hydroxide, and heating the resultant product at 300° C. for Al(OH) ₃ and at 600° C. for Cr(OH) ₃ or Mg(OH) ₂ in air.

[0038] The electrochemical performances of LiMn ₂ O ₄ and the surface/chemically modified LiMn ₂ O ₄ spinel oxide powders at both ambient and elevated temperatures are evaluated with coin cells. Cathodes are fabricated with the surface/chemically modified or unmodified LiMn ₂ O ₄ powder, Denka black carbon, and polytetrafluoroethylene (PTFE) binder in a weight ratio of 75:20:5. The coin cells (CR2032) may be assembled with the cathodes thus fabricated, metallic lithium anodes, polyethylene separators, and 1 M LiPF ₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) electrolyte may be cycled at various current densities between the voltage range of approximately 3.5 to 4.3 V using a battery cycler (manufactured by Arbin Instruments, College Station, Tex.).

[0039] Results and Discussion

[0040] FIG. 1 compares first and 100th discharge profiles of a LiMn ₂ O ₄ and a Li _x CoO ₂ -modified LiMn ₂ O ₄ cathodes at room temperature at 0.5 mA/cm ² , which corresponds to C/2 discharge rate. The discharge curves illustrated in FIG. 1 are labeled as follows: (a) LiMn ₂ O ₄ (cycle 1), (b) LiMn ₂ O ₄ (cycle 100), (c) LiCoO ₂ modified LiMn ₂ O ₄ (cycle 1), and (d) LiCoO ₂ modified LiMn ₂ O ₄ (cycle 100). The surface/chemically modified LiMn ₂ O ₄ exhibits better capacity retention compared to unmodified LiMn ₂ O ₄ . FIG. 2 compares the cyclability data of LiMn ₂ O ₄ with those of a number of surface/chemically modified LiMn ₂ O ₄ cathodes (with LiCo _0.5 Ni _0.5 O ₂ , LiCoO ₂ and Li _0.75 CoO ₂ ) up to 100 cycles at a current density of 0.5 mA/cm ² (C/2 rate) at room temperature. The cyclability data illustrated in FIG. 2 are labeled as (a) LiMn ₂ O ₄ , (b) LiCo _0.5 Ni _0.5 O ₂ -modified LiMn ₂ O ₄ , (c) LiCoO ₂ -modified LiMn ₂ O ₄ , and (d) Li _0.75 CoO ₂ -modified LiMn ₂ O ₄ . As evident from FIGS. 1 and 2 , the surface/chemically modified LiMn ₂ O ₄ compositions exhibit excellent cyclability. The percentage capacity fading (over 100 cycles) calculated from the discharge capacity values are given in Table 1. Among all the materials examined, the LiCo _0.5 Ni _0.5 O ₂ -modified LiMn ₂ O ₄ exhibits superior performance with a capacity fading value of less than 0.02% per cycle over 100 cycles. However, the LiCo _0.5 Ni _0.5 O ₂ -modified LiMn ₂ O ₄ sample exhibits lower initial capacity (111 mAh/g) than the unmodified LiMn ₂ O ₄ (127 mAh/g) as seen in Table 1. Additionally, the Li _0.75 CoO ₂ modified sample shows a higher capacity (123 mAh/g) than the LiCoO ₂ -modified sample (118 mAh/g). 1

[0041] FIG. 3 compares the cyclability data collected with a current density of 0.5 mA/cm ² (C/2 rate) at room temperature. The cyclability data illustrated in FIG. 3 are labeled as (a) LiMn ₂ O ₄ , (b) MgAl ₂ O ₄ -modified LiMn ₂ O ₄ , (c) Cr ₂ O ₃ -modified LiMn ₂ O ₄ , (d) Al ₂ O ₃ -modified LiMn ₂ O ₄ and (e) MgO-modified LiMn ₂ O ₄ . As seen from Table 1, Al ₂ O ₃ modified LiMn ₂ O ₄ material exhibits higher initial capacity (131 mAh/g) with a least capacity fading of less than 0.05% per cycle over 100 charge/discharge cycles, compared to other materials.

[0042] FIG. 4 compares the cyclability data collected at 60° C. with a current density of 0.5 mA/cm ² (C/2 rate). The cyclability data illustrated in FIG. 4 are labeled as (a) LiMn ₂ O ₄ , (b) LiCo _0.5 Ni _0.5 O ₂ -modified LiMn ₂ O ₄ , (c) LiCoO ₂ -modified LiMn ₂ O ₄ , (d) Li _0.75 CoO ₂ -modified LiMn ₂ O ₄ , and (e) Al ₂ O ₃ -modified LiMn ₂ O ₄ . The surface/chemically modified LiMn ₂ O ₄ cathodes show a higher capacity retention compared to that of unmodified LiMn ₂ O ₄ . As seen in Table 1, the unmodified LiMn ₂ O ₄ cathode shows a capacity fading of 0.41% per cycle over 100 cycles while the surface/chemically modified cathodes show a much lower fade rate. Among all the surface/chemically modified samples, the LiCo _0.5 Ni _0.5 O ₂ -modified LiMn ₂ O ₄ cathode exhibits the lowest fading rate of less than 0.03% per cycle over 100 cycles at 60° C. Among the various materials listed in Table 1, the Li _0.75 CoO ₂ modified LiMn ₂ O ₄ may provide the best combination of high capacity and good cyclability.

[0043] FIG. 5 compares the cyclability data of LiMn ₂ O ₄ and the surface/chemically modified LiMn ₂ O ₄ cathodes at a higher current density of 2 mA/cm ² (2C rate) at room temperature. The cyclability data illustrated in FIG. 5 are labeled as (a) LiMn ₂ O ₄ , (b) LiCo ₀ . ₅ Ni _0.5 O ₂ -modified LiMn ₂ O ₄ , and (c) LiCoO ₂ -modified LiMn ₂ O ₄ . As evident from FIG. 5 , the LiCoO ₂ -modified LiMn ₂ O ₄ exhibits excellent cyclability and rate capability at room temperature. The percentage capacity fading (over 100 cycles) calculated from the discharge capacity values at 2 mA/cm ² (2C rate) are given Table 1.

[0044] FIG. 6 compares the X-ray diffraction patterns of LiMn ₂ O ₄ and the surface/chemically modified LiMn ₂ O ₄ spinel cathodes in discharged state after cycling at 60° C. over 100 cycles. The X-ray diffraction patterns illustrated in FIG. 6 are labeled as follows: (a) LiMn ₂ O ₄ cathode, (b) Li _0.75 CoO ₂ modified LiMn ₂ O ₄ cathode, (c) Al ₂ O ₃ modified LiMn ₂ O ₄ cathode, (d) LiCoO ₂ modified LiMn ₂ O ₄ cathode, and (e) LiCo _0.5 Ni _0.5 O ₂ modified LiMn ₂ O ₄ cathode. As seen from FIG. 6 , unlike the surface/chemically modified LiMn ₂ O ₄ , the unmodified LiMn ₂ O ₄ spinel cathode shows peak broadening indicating structural degradation during cycling at elevated temperatures. Similar results are also found for samples soaked in the electrolyte (1M LiPF ₆ in EC and DEC) at 55° C. The peak-broadening feature (loss of crystallinity) could be due to the degradation of the particles of LiMn ₂ O ₄ spinel. It is generally known that the crystallinity decreases proportionately with the extent of capacity fading. The surface/chemical modification of LiMn ₂ O ₄ appears to protect the LiMn ₂ O ₄ crystals from attack by the acidic species contained in the electrolyte and thereby leads to the maintenance of the well-defined crystallites during cycling. It may be theorized that the capacity fading of LiMn ₂ O ₄ at elevated temperatures is due to the loss of active material from the surface during cycling.

[0045] Transmission electron microscopic (TEM) studies indicate that while the firing at elevated temperatures of around 800° C. leads to a diffusion of the surface modification material into the bulk of the electrode material, the firing at lower temperatures of around 300° C. leads to the presence of a significant amount of the surface modification material on the surface. So the former and latter cases may be termed as chemical modification and surface modification respectively. Thus the process described in this invention may broadly be considered as either surface modification or chemical modification or both depending upon the final firing temperature.

Example 2

Surface-Modified LiCoO ₂ Cathodes

[0046] A commercially available LiCoO ₂ powder may be used as the electrode material. FIG. 7 compares the cyclability data of LiCoO ₂ and Al ₂ O ₃ -modified LiCoO ₂ at room temperature and at 60° C. in various voltage ranges of 4.3-3.2 and 4.5-3.2 V at C/5 rate. The cyclability data illustrated in FIG. 7 are labeled as (a) LiCoO ₂ at 25° C. (4.3-3.2 V), (b) LiCoO ₂ at 25° C. (4.5-3.2 V), (c) Al ₂ O ₃ -modified LiCoO ₂ at 60° C. (4.3-3.2 V), (d) Al ₂ O ₃ -modified LiCoO ₂ at 25° C. (4.5-3.2 V), and (e) Al ₂ O ₃ -modified LiCoO ₂ at 60° C. (4.5-3.2 V).

[0047] The data reveals that LiCoO ₂ suffers from capacity fading severely ( FIG. 7 ( a )) when the charging cut-off voltage is increased to 4.3 V at 25° C. LiCoO ₂ cathodes are conventionally cycled up to a charging cut-off voltage of 4.2 V with a capacity of around 140 mAh/g and below 4.2 V it is known to cycle well. On the other hand, Al ₂ O ₃ -modified LiCoO ₂ does not show any fading during cycling with a voltage range of 4.3-3.2 V. FIG. 7 also shows the cyclability data at 60° C. in the voltage range of 4.3-3.2 V. The Al ₂ O ₃ -modified LiCoO ₂ does not show any capacity fading during cycling in the voltage range of 4.3-3.2 V at even 60° C. FIG. 7 also shows the cyclability data for Al ₂ O ₃ -modified LiCoO ₂ at room temperature and at 60° C. in the voltage range of 4.5-3.2 V. The Al ₂ O ₃ -modified LiCoO ₂ exhibits very good cyclability at elevated temperatures with very little capacity fading. The data show that the unmodified LiCoO ₂ cathode exhibits severe capacity fade in the voltage range of 4.5-3.2 V. The good cylability of the Al ₂ O ₃ -modified LiCoO ₂ up to a charging cut-off voltage of 4.5 V enables to achieve a much higher capacity of around 190 mAh/g compared to the 140 mAh/g generally achieved with unmodified LiCoO ₂ cathode.

[0048] The capacity fading of unmodified LiCoO ₂ at higher voltages could be due to the loss of oxygen and dissolution of cobalt from the lattice. The surface modification with Al ₂ O ₃ seems to suppress these problems and improve the capacity retention at higher cut-off charge voltages. Transmission electron microscopic (TEM) studies show that the Al ₂ O ₃ is present on the surface of LiCoO ₂ .