Nano porous battery electrode material

Technical field

The invention relates to a method of producing a nano porous battery electrode material and a nano porous battery electrode material.

Background and related art

Batteries belong to the most important power sources which are used in different areas of operation. Almost any electrical consumer can be equipped with batteries in order to use electrical energy which results from discharging of the battery through an electrochemical redox reaction.

Lithium is a widespread negative electrode material for batteries. This is due to the fact that lithium has the most negative standard potential of all elements which allows realizing obtaining high battery cell voltages. Also, using lithium theoretically extremely high battery capacities are accomplishable. Indeed, since many years suitable electrode materials are developed for uptaking and releasing of lithium ions in combination with respective electrolyte materials in order to achieve such high theoretical energy densities of lithium batteries in practice. One electrode material which can be used to realize such high discharge voltages while maintaining a high capacity are lithium phosphor compounds in the form of olivines, as for example LiMPO ₄ , wherein M Is a metal like iron, manganese, cobalt etc.

Phospho-olivines such as LiMPO ₄ (M = transition metal) are attractive candidates for cathode materials in lithium ion batteries, because of their excellent cyclability, thermal stability, low cost and environmental benefits. However, these materials suffer from a low electronic and ionic conductivity of LiMPO ₄ leading typically to poor rate capabilities. Immense technical efforts have therefore been devoted to counteract this problem, one approach being the synthesis of well dispersed and small particles to shorten the diffusion path length of lithium ions.

For example, J. Electrochem. Soc, Vol. 144, No. 4, April 1997, p. 1188 - 1194 discloses that phospho-olivine is a positive electrode material suitable for rechargeable lithium batteries.

Similarly, J. Electrochem. Soc, Vol. 148, No. 8, A960 - A967, 2001 deals with the usage of olivine type lithium compounds as a possible cathode material for lithium batteries.

US 5,910,382 discloses the usage of transition metal compounds with an ordered olivine or rhombohedral Nasicon structure as electrode material for rechargeable alkali ion batteries.

R. Dominko, M. BeIe, M. Gaberscek, M. Remskar, D. Hanzel, J. M. Goupil, S. Pejovnik, J. Jamnik, "Porous olivine composites synthesized by sol-gel technique", J. Power Sources, 2006, 153, 274-280 does disclose porous UMPO ₄ /C composites (where M stands for Fe and/or Mn) with micro-sized particles which were synthesized by a sol-gel technique. The document discusses porosity in terms of qualitative results obtained from SEM micrographs and in terms of quantitative results obtained from N ₂ adsorption isotherms. Porous particles are described as an inverse picture of nano particulate arrangements, where the pores serve as channels for lithium supply and the distance between the pores determines the material kinetics.

Summary of the invention

Nanoporous carbon structures with a monomodal pore size distribution, thus meaning that all pores display almost the same pore size, such as CMK-basβd materials have been prepared via templating approaches already a couple of years ago. For example such a method is dislosed in Che Shunai; Lund Kristina; Tatsumi Takashi; lijima Sumio; Joo Sang Hoon; Ryoo Ryong; Terasaki Osamu Angew. Chem. Int. Ed. 2003, 42, 2182-5.

A bimodal porous carbon structure comprising macropores and mesopores regularly aligned is provided. A bimodal distribution most commonly arises as a mixture of two different monomodal distributions, in this case pore sizes. A method for preparation of a bimodal porous carbon structure having uniform (monomodal) macropores and monomodal mesopores, thus with a very narrow pore size distribution, three- dimensionally connected to each other and regularly aligned has been reported in Lu, An-Hui; Schmidt, Wolfgang; Spliethoff, Bernd; Schϋth, Ferdi. Adv. Mater. 2003, 15, 1602-1606.

Carbon-based materials have already been reported as templates in the synthesis of nanoscale materials that can be applied In electrochemical applications. As one example, Du et al. report the synthesis of an active material (Co ₃ O ₄ nanotubes) on carbon-nanotube templates and their applications in Li-ion batteries (N. Du ₁ H. Zhang, el. Adv. Mater. 2007, 19, 4505-09).

The present invention provides a method of producing a nano porous battery electrode material, the method comprising providing an electrically conductive support consisting of a bimodal matrix structure, the matrix structure comprising nano and micro pores with a monomodal nano and micro pore size distribution. The method further comprises evacuating the support and soaking the evacuated support with precursor liquid, the precursor liquid comprising precursor components for a temperature-controlled synthesis of phosphoolivine nano crystals. The method further comprises heat treating the soaked support for formation of the phosphoolivine nano crystals within the nano pores.

The method according to the invention has the advantage, that a nano porous battery electrode material applicable for extremely high charge and discharge rates can be provided. Important is here the combination of nano- and micron-sized pores.

For example, for usage in a battery, an electrolyte needs to be infiltrated into the micro pores of the electrode material. Even though, a pure nano porous material would give good electronic and ionic contacts, a diffusion of an electrolyte would be hindered due to high capillary pressures in case only nanopores would be present in the electrode material. Therefore, by using a mixture of nano- and micro pores this problem can be solved.

Further, due to the nanometre size distribution of nano pores, the growth of phosphoolivine nano crystals is limited to the size of these nanopores. Thus, by performing the step of heat treating the soaked support will yield in a well defined manner a nano sized network of phosphoolivine crystals which provide small diffusion times of lithium and therefore enhanced electrical conductivities.

In summary, by the combination of a monomodal nano- and micropore distribution, the features of a finely distributed phosphoolivine crystal 'network' and an electrically conductive support which can be easily soaked due to the micro sized pores by an electrolyte, a large number of active triple phase contacts can be provided which is highly advantageous for high performance battery electrode materials. The microscopic network of the electrically conductive support will thus provide high battery charging capacities and will also allow for a quick infiltration of electrolytes.

Preferably, the nano pores have a diameter in the range of 1-20 nm and the micropores have a diameter distribution in the range of 200 nm up to 2 μm.

In accordance with an embodiment of the invention, the precursor components comprise a dissolved lithium salt, a dissolved transition metal salt and a phosphor source. For example, the phosphoolivine nano crystals may comprise a material with the composition Li _x MyPO ₄ with M=Ti, V, W, Cr ₁ Mn, Fe, Co, Nl, Cu, Mg, Ca, Sr, Pb, Cd, Ba ₁ Be and/or may comprise material of the composition U _x Fei. _y Ti _y PO ₄ and/or Li _x Fei. _y Mn _y PO ₄ with 0<y<1. It has to be noted here, that it is important that the precursors are completely dissolved in a liquid like for example water such that by evacuation of the support the precursor liquid can be homogeneously sucked into the nanopores of the electrically conductive support. For example, for this purpose lithium acetate dihydrate and manganese acetate tetrahydrate are dissolved in water and mixed with phosphoric acid as phosphor source. Alternatively, as phosphor source also ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) or other phosphates could be used.

In accordance with an embodiment of the invention, the precursor components further comprise a chelating agent. For this purpose, for example glycolic acid or other chelating carboxylic acids can be additionally added to the above mentioned precursor components. The usage of an additional chelating agent has the advantage, that the reaction kinetics can be efficiently controlled. This speeds up the reaction process which is an important aspect for industrial scale production processes.

Regarding the usage of preferably phosphoric acid as phosphor source it has to be mentioned, that the usage of an acid has further the advantage, that the pH value of the precursor liquid can be efficiently controlled in order to prevent an unwanted precipitation of solids.

In accordance with an embodiment of the invention, the support is a carbon support, for example ordered nanoporous carbon CMK-3 or CMK-5, preferably nanoporous carbon aerogels, or a similar material with a bimodal pore size distribution. The usage of a carbon support has the advantage, that a highly chemically, mechanically, electrically and thermodynamlcally resistant material is used, which will thus not limit the application of a nanoporous battery electrode material with respect to applied electrolytes, operating environment even at high temperatures and high electrical currents.

However, in general any other material could be used as support, with the constraints that the material is conductive and comprises a bimodal matrix structure. Another constraint is, that the material can withstand the heat treatment step without substantial degradation. Preferably, further said material should contain a carbon source which could be used as assistance for the formation of the phosphoolivine nano crystals. For example, the support may also be a metallic carbide support, for example comprising molybdenum carbide.

In accordance with an embodiment of the invention, the carbon support comprises carbon nanofibres and/or carbon nanotubes arranged in a three-dimensionally connected fashion displaying porosity on two size levels. The usage of carbon nanofibres and/or carbon nanotubes has the advantage, that due to Van-der-Waals forces carbon nanofibres and/or carbon nanotubes self-arrange themselves in for example hexagonal bundled structures which thus provide interstitial sites as nano pores for receipt of the precursor liquid and/or an electrolyte when used as battery electrode material. The sites in between entangled bundles of bundled carbon fibers or bundled carbon nanotubes provide the micro pores.

In accordance with an embodiment of the invention, the evacuation and soaking steps are repeated multiply. This provides a homogeneous and almost complete coating of the matrix structure with phospho-olivlne nanocrystals within the nanopores.

In accordance with an embodiment of the invention, the heat treating is performed by hydrothermal treatment and/or ambient conditions treatment and/or microwave treatment. However, a hydrothermal treatment is performed preferably since this drastically speeds up the reaction kinetics such that also In a commercially applicable timeframe a production of said nanoporous battery electrode material is possible.

In another aspect, the invention relates to a nano porous battery electrode material, the electrode material comprising an electrically conductive support, the support consisting of a bimodal matrix structure, the matrix structure comprising nano and micron-sized pores with a monomodal nano and micro pore size distribution. The electrode material further comprises phosphoolivine crystals being contained within the nanopores.

In accordance with an embodiment of the invention, the phosphoolivine nano crystals have a diameter smaller than 20 nm.

Brief description of the drawings

In the following, preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which:

Figure 1 is a schematic illustrating the method according to the invention of producing a nanoporous battery electrode material,

Figure 2 shows various photographs of the electrically conductive support used for production of the nanoporous battery electrode material,

Figure 3 shows various SEM and TEM pictures of the electrically conductive support before and after soaking and heat treating with the precursor liquid,

Figure 4 is a flowchart illustrating the method of producing a nanoporous battery electrode material,

Figure 5 shows a Hg porosimetry measurement performed on a porous carbon support displaying the macroporous nature of the material

Figure 6 shows various XRD measurement results performed on various nano porous battery electrode materials. Detailed description

Fig. 1 illustrates the method of producing a nanoporous battery electrode material according to the invention. The method of producing a nanoporous battery electrode material comprises various steps: in a first step, an electrically conductive support 100 is provided, wherein a support consists of a bimodal matrix structure comprising nano- and micron-sized pores with a monomodal nano and micropore size distribution. In the example illustrated in fig. 1 , the matrix structure is given by a carbon fiber bundle 101 consisting of a multitude of individual carbon nanofibers 104 which are hexagonally arranged. In between the hexagonally arranged carbon nanofibers 104, interstitial sites 106 as nanopores are provided. Micropores which can be filled with an electrolyte are formed by for example an entanglement and thus an interwoven network of individual carbon fiber bundles 101.

By means of an impregnation step 108, a precursor liquid comprising precursor components for heat control synthesis of phosphoolivine nano crystals can be filled into the pores 106 of the electrically conductive support 100. Heat treatment of the soaked support 102 will lead to the formation of phosphoolivine nano crystals 110 within the nanopores of the conductive support 100.

As a carbon source, the template 100 is not restricted to bundles of carbon nanofibers and/or nanotubes. Many kinds of hierarchically organized carbon monoliths can be used. Additionally, carbon-based inverse opal structures, carbon aerogels etc. can be used.

Preferably, the precursor mixture used for impregnation in step 108 is based on the highly water soluble acetates of manganese and lithium that were dissolved in a derivative of phosphoric acid and glycolic acid or another chelating agent.

Fig. 2 shows various pictures (photograph, SEM image and TEM images) of carbon monoliths which were used as electrically conductive support for producing a nanoporous battery electrode material according to the invention. For example Fig, 2a shows an optical photograph of a carbon monolith which has a mechanically rigid macroscopic cylindrical structure with a diameter of about 3mm and a length of about 3cm.

Fig. 2b shows a TEM (transmission electron microscope) image of this carbon monolith which clearly shows the fibrous structure of the monolith material. Fig. 2d shows a further increased magnification of said material at a different view angle such that the distal ends of the individual fibers appear in their hexagonal structure in the photograph. From fig. 2d it can be clearly seen, that due to the hexagonal structure of the individual fibers interstitial sites between the individual fibers are provided which have a monomodal size distribution in the nanoscopic length scale.

The co-existing micropores in the electrically conductive support can be seen in fig. 2c which is an SEM (scanning electron microscopy) image. Individual bundles of fibers are entangled and percolating such that in between the individual bundles micropores exist which can be easily soaked and filled by electrolytes.

Fig. 3 shows various TEM images of a native carbon monolith (fig. 3a and 3c) and a carbon monolith after impregnation with LiMPO ₄ (figs. 3b and 3d). In fig. 3a which is a TEM image of a native carbon monolith, the micropores 300 in between the entangled individual carbon fibers can be clearly seen which are easily penetrable by a respective electrolyte. Fig. 3c is a TEM image of the same native carbon monolith, however with higher magnification. Also here, a micropore 300 is visible. Further, individual carbon fibers can be seen which themselves form nanopores 302, similarly to the nano pores explained with respect to fig. 2d.

After impregnation, a filling of interstitial sites of nano pores formed by neighboring individual carbon fibres with phosphoolivine nano crystals of the composition LiMnPO ₄ is observed. In the TEM picture of fig. 3d a fiber bundle with its interstitial sites filled with phosphoolivine nano crystals 304 can be seen. The phosphoolivine nano crystals have a typical size which is smaller than 20nm, preferably smaller than 5nm. Rg. 4 is a flowchart illustrating the method of producing a nanoporous battery electrode material according to the invention. In step 400, precursor components for example manganese acetate tetrahydrate and lithium acetate dihydrate are dissolved in water. Further, phosphoric acid as phosphor source is added.

Step 402 which comprises adding a chelating agent, for example glycolic acid, is optional but preferred, since the addition of a chelating agent further speeds up the reaction kinetics.

In step 404, the electrically conductive support, for example a carbon monolith is evacuated and thereafter in step 406 impregnated, i.e. soaked with the precursor liquid prepared in steps 400 and 402. Next, the impregnated carbon material is heated for drying purposes in step 408. Thereafter, step 410 is carried out which comprises a calcination procedure and thus subsequent formation of lithium manganese phosphate, i.e. phosphoolMne nano crystals. For example, the calcination in step 410 can be performed by heating the impregnated sample gradually up to high temperatures above 550 ⁰ C under inert atmosphere.

After step 408, the steps 404, 406 and 408 can be performed repeatedly again, wherein each repetition increases the coating of the conductive support matrix structure with phospho-olivine nano crystals. Alternatively, the repetition can be performed including step 410 in every repetitive cycle.

Fig. 4 is a mercury porosimetry measurement of a nano/ microporous carbon monolith. It can clearly be seen that the pores in the microscopic length scales are very equivalent in size, thus monomodal, with a maximum in the size distribution close to 500 nm in diameter.

The improved coverage of the carbon monolith matrix structure by phosphoolivine nano crystals with repeated Impregnation of the carbon monolith is illustrated in fig. 6 which shows XRD (X-ray diffraction) images of a carbon monolith at different impregnation states. In fig. 6a, an XRD measurement was performed on a one-time impregnated carbon monolith. In fig. 6b, the carbon monolith was two-times impregnated and in fig. 6c, the carbon monolith was three-times impregnated. It can be clearly seen, that from fig. 6a to fig. 6b to fig. 6c, the peaks indicated by the arrows which belong to LiMnPO ₄ are increasingly stronger pronounced with increasing number of impregnation steps.

List of Reference Numerals

100 conductive support

102 conductive support comprising LiMPO ₄

104 fiber

106 nano pore

110 olivine nano crystal

300 micro pore

10 302 nano pore

304 olivine nano crystals