Abstract:
A transition metal hydroxy-anion electrode material for lithium-ion battery cathodes includes the charge-neutral structure M x (OH) n (XO4) m , where M is one or more transition metals, x is the total number of transition metal atoms, X is sulfur or phosphorus, and x, n, and m are integers. (OH) n (XO 4 ) m  is a hydroxysulfate or hydroxyphosphate, and M can be one or more (e.g., a solid solution of) transition metals selected from the group consisting of copper, iron, manganese, nickel, vanadium, cobalt, zinc, chromium, and molybdenum. A lithium-ion battery may have a cathode including M x (OH) n (XO 4 ) m  as a cathode material, and an electronic device may include a lithium-ion battery having a cathode including M x (OH) n (XO 4 ) m  as a cathode material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. App. Ser. No. 61/662,134 entitled “Transition Metal Hydroxy-Anion Electrode Materials for Lithium-Ion Battery Cathodes” and filed on Jun. 20, 2012, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to transition metal hydroxy-anion electrode materials for lithium-ion battery cathodes. 
       BACKGROUND 
       [0003]      FIG. 1A  depicts lithium-ion battery (LIB)  100  having anode  102  and cathode  104 . Anode  102  and cathode  104  are separated by separator  106 . Anode  102  includes anode collector  108  and anode material  110  in contact with the anode collector. Cathode  104  includes cathode collector  112  and cathode material  114  in contact with the cathode collector. Electrolyte  116  is in contact with anode material  110  and cathode material  114 . Anode material  110  and electrolyte  116  are generally known in the art. Anode collector  108  and cathode collector  112  are electrically coupled via closed external circuit  118 . Anode material  110  and cathode material  114  are materials into which, and from which, lithium ions  120  can migrate. During insertion (or intercalation) lithium ions move into the electrode (anode or cathode) material. During extraction (or deintercalation), the reverse process, lithium ions move out of the electrode (anode or cathode) material. When a LIB is discharging, lithium ions are extracted from the anode material and inserted into the cathode material. When the cell is charging, lithium ions are extracted from the cathode material and inserted into the anode material. The arrows in  FIG. 1A  depict movement of lithium ions through separator  106  during charging and discharging.  FIG. 1B  depicts device  130  including LIB  100 . Device  130  may be, for example, an electric vehicle, an electronic device (e.g., a portable electronic device such as a cellular telephone, a tablet or laptop computer, etc.), or the like. 
         [0004]    High capacity and high rate LIBs with low cost and improved safety characteristics constitute a major requirement for electric vehicles, portable electronics, and other energy storage applications. Year-to-year electrochemical performance improvements in LIBs are typically limited to 3-4%, with a major bottleneck being the lack of appropriate materials to satisfy the energy and power density requirements. Progress in nanostructured anodes has significantly improved the potential of the practically achievable capacity and rates. For example, high capacity anodes such as silicon, which have been studied since the 1980s, have been found to overcome structural degradation problems through the use of nanowire morphologies. However, batteries utilizing silicon anodes can still only achieve a 30% gain in energy density due to the low capacity of the cathode: current cathodes have practical capacities of 150-180 mAh/g. While nanostructuring of existing cathodes has been found to lead to improvements in usable charge capacity and result in higher rate performance, the theoretical capacities of existing materials is still too low. 
       SUMMARY 
       [0005]    Transition metal hydroxyl anion materials including M x (OH) n (XO 4 ) m , in which M is a transition metal (e.g., Cu, Fe, Mn, Ni, V, Co, Zn, Cr, Mo, and solid solutions thereof), x is the total number of transition metal atoms, and X is S or P (such that the anion is a hydroxysulfate or hydroxyphosphate), are described for use in lithium-ion battery cathodes. These transition metal hydroxysulfate and hydroxyphosphate anions demonstrate improved performance as cathode materials based at least in part on characteristics such as (i) an open framework or layered structure that facilitates fast lithium ion insertion; (ii) beneficial bonding characteristics such as edge-sharing MO 6  octahedra for good electronic conductivity and improved rate performance; (iii) flexibility in alkali and transition metal cation incorporation, which can allow for the design of solid-solutions to enhance structural stability, capacity, and reaction potentials; and (iv) possibility for multielectron redox reactions due to the incorporation of more than one transition metal per formula unit, which can result in capacities exceeding 200 mAh/g. 
         [0006]    In a first general aspect, an electrode for a lithium-ion battery includes a polyanion material including M x (OH) n (XO 4 ) m , where M is one or more transition metals, x is the total number of transition metal atoms, X is sulfur or phosphorus, and x, n, and m are integers. In a second general aspect, forming an electrode for a lithium-ion battery includes preparing a composition including a polyanion material including M x (OH) n (XO 4 ) m , and contacting the composition with a current collector to form the electrode. 
         [0007]    Implementations may include one or more of the following features. The electrode including the polyanion material may be a cathode for a lithium-ion battery. M may be selected from the group consisting of copper, iron, manganese, nickel, vanadium, cobalt, zinc, chromium, molybdenum, and any combination thereof. In some cases, M is a solid solution of two or more transition metals selected from the group consisting of copper, iron, manganese, nickel, vanadium, cobalt, zinc, chromium, and molybdenum. In certain cases, M includes at least two transition metals or x is at least 2. The polyanion material may be a hydroxysulfate or a hydroxyphosphate. The polyanion material may have edge-sharing octahedra and a non-tavorite structure. The polyanion material may include Li a M x (OH) n (XO 4 ) m , where a is an integer. 
         [0008]    In some implementations, a lithium-ion battery includes the electrode of the first general aspect and/or the second general aspect. The lithium-ion battery further includes an anode and an electrolyte in contact with the anode and the cathode, as generally known in the art. In some cases, the lithium-ion battery has a capacity of at least 200 mAh/g. In certain implementations, a device (e.g., an electronic device) includes a lithium-ion battery including the electrode of the first general aspect and/or the second general aspect, or any implementation thereof. 
         [0009]    These general and specific aspects may be implemented using a device, system or method, or any combination of devices, systems, or methods. The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  depicts a lithium-ion battery (LIB).  FIG. 1B  depicts a device including a LIB. 
           [0011]      FIG. 2A  shows plots of potential (vs. Li/Li + ) versus capacity (mAh/g) for the reaction of Cu 2 (OH)PO 4  (libethenite) with lithium as described in Example 1.  FIG. 2B  shows a scanning electron microscope (SEM) image of Cu 2 (OH)PO 4  nanorods.  FIG. 2C  shows an X-ray diffraction pattern of Cu 2 (OH)PO 4  nanorods. 
           [0012]      FIG. 3A  shows plots of potential (vs. Li/Li + ) versus capacity (mAh/g) for the reaction of Cu 4 (OH) 6 SO 4  (brochantite) with lithium as described in Example 2.  FIG. 3B  shows an SEM image of Cu 4 (OH) 6 SO 4  particles.  FIG. 3C  shows an X-ray diffraction pattern. Cu 4 (OH) 6 SO 4  particles. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    As described herein, lithium-ion battery (LIB) cathodes including transition metal hydroxyl anion materials having the charge-neutral structure M x (OH) n (XO 4 ) m , in which M is a transition metal (e.g., Cu, Fe, Mn, Ni, V, Co, Zn, Cr, Mo, and solid solutions thereof), X is S or P (such that the anion is a hydroxysulfate or hydroxyphosphate), and x, n, and m are integers provide a desired combination of high charge storage capacity and structural stability. These polyanion materials provide an open framework or layered structure with interstitial spaces that can accommodate lithium ions as well as different transition metals, thereby allowing tuning of redox potentials and capacities. Unlike tavorite hydroxyl anion materials, the metal hydroxysulfate and hydroxyphosphate materials including M x (OH) y (XO 4 ) n  are non-tavorite structures, and thus have edge-sharing rather than corner-sharing octahedra. 
         [0014]    Metal hydroxysulfate and hydroxyphosphate materials of the form M x (OH) y (XO 4 ) n  occur in a variety of expanded frameworks and layered structures Examples include Cu 2 (OH)PO 4  (libethenite), Cu 3 (OH) 3 PO 4  (cornetite) and Cu 5 (OH) 4 (PO 4 ) 2  (psuedomalachite), Cu 4 (OH) 6 SO 4  (brochantite), Cu 3 (OH) 4 SO 4  (antlerite), Cu 6 (OH) 10 SO 4  (montetrisaite), and sodium iron (III) hydroxyphosphate. The synthesis of hydroxysulfate and hydroxyphosphate materials is facilitated by the fact that the compounds can be precipitated from aqueous solutions of the metal salts or synthesized using hydrothermal methods. For example, the copper hydroxyphosphate libethenite is generally understood to be formed by mixing Cu(NO 3 ) 2  and (NH 4 ) 2 HPO 4  and precipitating in acidic solutions; the copper hydroxysulfate brochantite are generally understood to be synthesized by refluxing CuCl 2  in NH 4 SO 4  and NaOH. Copper hydroxysulfates and hydroxyphosphates can also be obtained by hydrothermal reaction of CuSO 4  in NaOH or H 3 PO 4 , respectively. The antlerite and brochantite compositions can be obtained by changing the Cu/NaOH/H 2 O molar proportions. When these materials are synthesized in nanoparticle form, the resulting nanostructured morphology is believed to improve electrochemical performance due at least in part to the decreased distance required for electronic transport and lithium ion diffusion. 
         [0015]    In one example, Cu 2 (OH)PO 4 , the mineral libethenite, includes PO 4  tetrahedra, CuO 4 (OH) trigonal bipyramids, CuO 4 (OH) 2  octahedra, and OH groups linking the two Cu species. The structure has chains of edge-sharing CuO 4 (OH) 2  octahedra parallel to the c-axis, but no P—O—P chains, which imparts good electronic conductivity. As described herein, Cu 2 (OH)PO 4  displays electrochemical activity and can reversibly intercalate Li +  ions. The theoretical capacity based on the Cu 2+/1+  couple is 224 mAh/g, which is significantly higher than the ˜150 mAh/g observed for LiCoO 2  and LiMn 2 O 4  and the 120-170 mAh/g for other polyanion cathodes (e.g., LiFePO 4 , LiFeSO 4 F). The higher capacity is understood to be due to the presence of two transition metal ions per formula unit, resulting in the 2e −  process: 
         [0000]      Cu 2 (OH)PO 4 +2e − +2Li + →Li 2 Cu 2 (OH)PO 4 .
 
         [0000]    For reduction of the Cu 2+  to Cu 0 , as in a conversion reaction, the theoretical capacities (for the 4e −  process) would increase to 448 mAh/g. In some cases, lithiated materials having the structure Li a M x (OH) n (XO 4 ) m  as described herein may be synthesized directly. 
         [0016]    While libethenite does not contain alkali ions in its initial state, there are other hydroxyphosphate materials that are found in nature already with alkali ions incorporated. For example, sodium iron hydroxyphosphate (SIHP) has a formula Na 3 Fe(PO 4 ) 2 .Na 2(1-x) H 2x O, where 0.2&lt;x&lt;0.4. The structure has Fe—O—Fe chains with two phosphates linking adjacent iron atoms. The bridging hydroxyl groups can associate with H +  or Na +  cations, which are located in the relatively open channels of the phosphate lattice. Thus, this structure has the Fe—O—Fe bonding required for good electronic conductivity, in addition to the Fe—O—X—O—Fe (where X=PO 4 ) bonding which will promote higher voltages. The open channels may also promote good diffusion of Na +  ions. 
         [0017]    Hydroxysulfate materials also display interesting structures that may promote high electronic and ionic conductivities. For example, the Cu hydrokysulfate family consists of edge-shared Cu octahedra that form layers. These layered structures may promote the fast insertion/deinsertion of Li + . The theoretical capacities for Cu 3 (OH) 4 SO 4  (antlerite), Cu 4 (OH) 6 SO 4  (brochantite), and Cu 6 (OH) 10 SO 4  (montetrisaite) for the 3, 4, and 6 electron reduction processes are 227, 237, and 248 mAh/g, respectively. As with the hydroxyphosphates, some iron hydroxysulfates exist already containing alkali metals. For example, solid solutions of the form Li x K 1-x Fe 3 (OH) 6 (SO 4 ) 2 , similar to the mineral jarosite (MFe 3 (OH) 6 (SO 4 ) 2 , with M=Na, K, Rb, NH 4 , Ag), have been observed. 
         [0018]    The selection of related hydroxyanion materials that may be electrochemically active can be guided by the typical redox potentials of transition metals used in cathode materials for LIBs, such as the Cu 2+/1+ , Fe 3+/2+ , and Mn 3+/2+ , V 4+/3+ , and Co 3+/2+  redox couples, as well as the existence of stable compositions and structures from mineralogy. Because these materials are based on naturally occurring minerals, the oxidation states of the transition metals are typically in the commonly found +2 valence. For example, there are Mn 2+  and Ni 2+  hydroxysulfate analogs to the copper-containing libethenite, cornetite, etc., which may be suitable cathode materials for LIBs under conditions in which the M 3+/2+  couple is accessible. A lithiated jarosite of the form LiFe 3 (OH) 6 (SO 4 ) 2  may be delithiated upon oxidation to Fe 4+ . The V 3+  analog of jarosite is expected to have advantageous electrochemical properties, since the V 4+/3+  and V 5+/4+  couples are thought to be electrochemically accessible. 
         [0019]    These hydroxyanion materials may be made into solid solutions or mixed metal compounds, which can further affect the structural stability and voltage characteristics. For example, Cu 2+ Fe 3+ (OH)(SO 4 ) 2 .4H 2 O is known to exist as the mineral guildite, and the solid solution Co 2-x Cu x (OH)PO 4  has been made synthetically. The presence of multiple transition metals has been shown to modify the structure or improve Li diffusivity in other polyanion systems without being redox active (known as the bystander effect), suggesting that the electrochemical properties of these solid solutions may be tunable to achieve optimal voltage and capacity. 
         [0020]    Another attractive feature of polyanion materials is that because the anions are larger than O 2− , they can be more easily found in a variety of open framework structures that can facilitate the diffusion of Li + , and perhaps even larger cations such as Na + , Mg 2+ , and Ca 2+ . This may also impart an improved structural stability during cation de-intercalation. However, the heavier weight of the oxyanion lowers the gravimetric capacity, necessitating the use of multi-electron redox processes. 
         [0021]    As described herein, hydroxysulfate and hydroxyphosphate materials having edge-sharing (not corner-sharing) octahedra offer a flexible and tunable platform, in terms of composition and structure, having open framework or layered structures with space for lithium ions to intercalate. The structural tunability as well as unique bonding can offer improved electronic and ionic conductivities compared to other polyanion materials, which can affect the charge/discharge rates and power capabilities. Also, the presence of multiple transition metals per formula unit facilitates multi-electron redox reactions, which can lead to high capacity cathode materials. 
       EXAMPLES 
     Example 1 
       [0022]    A LIB cathode was prepared by mixing commercially available Cu 2 (OH)PO 4  powder (Sigma Aldrich) with 10 wt % carbon black and 10 wt % polyvinylidene difluoride (PVDF) binder in a slurry with N-methyl pyrrolidone as solvent, then coating as a film onto aluminum foil current collectors using a Meyer rod. A LIB was formed with a lithium metal anode and an electrolyte of 1 M LiPF 6  in 1:1 ethylene carbonate: diethylcarbonate. Charge/discharge curves  200 ,  202 ,  204 ,  206 , and  208  from five consecutive cycles with the LIB are show in  FIG. 2A , with the upper and lower portion of each plot corresponding to charge and discharge curves, respectively. As shown in  FIG. 2A , Cu 2 (OH)PO 4  was found to be electrochemically active and capable of reversibly intercalating lithium.  FIG. 2B  is a scanning electron microscope (SEM) image of the Cu 2 (OH)PO 4  nanorods  210  used to form the LIB cathode of this example. The nanorods are generally 1-2 μm in length and 100-200 nm in diameter.  FIG. 2C  shows an X-ray diffraction pattern  220  of the Cu 2 (OH)PO 4  nanorods. 
       Example 2 
       [0023]    Brochantite was synthesized using titration of 0.1 M NaOH into CuSO 4  of the same concentration, yielding particles in a range of 1-20 μm. A LIB was formed with a lithium metal anode and an electrolyte of 1 M LiPF 6  in 1:1 ethylene carbonate: diethylcarbonate. Charge/discharge curves  300 ,  302 ,  304 ,  306 , and  308  from five consecutive cycles with the LIB are show in  FIG. 3A , with the upper and lower portion of each plot corresponding to charge and discharge curves, respectively. As shown in  FIG. 3A , this material was found to be electrochemically active and capable of reversibly intercalating lithium. The large particle sizes may be responsible for the lower observed capacities than expected theoretically and the capacity fade with subsequent cycling. However, it is believed that this can be improved through nanostructuring the material.  FIG. 3B  shows an SEM image of the Cu 4 (OH) 6 SO 4  particles  310  used to form the LIB in this example.  FIG. 3C  shows an X-ray diffraction pattern  320  of Cu 4 (OH) 6 SO 4  particles. 
         [0024]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.