**1.3. Batteries and battery materials**

There are many battery systems available for consumer use, and over the years since commercial introduction, their performance has improved due to a combination of advances in material design and cell engineering [5]. The various battery chemistries that are available can be categorized as either primary (single use) or secondary (rechargeable) systems. The battery system that is currently receiving the most attention in the literature is based on the Li-ion chemistry. Here Li+ ions are reversibly shuttled between the positive and negative electrodes in the cell, which with the use of an appropriate non-aqueous electrolyte, can achieve a potential of over 4 V [5,6]. Some of the commonly found materials in Li-ion batteries include:


Whatever the combination of positive and negative electrode that is used, the performance of the resultant cell is quite significantly determined by the properties of the electroactive materials used, which in turn are determined by the way in which they were prepared. Material properties such as phase purity, crystallinity and particle size (or extent of agglomeration) all affect performance.

## **1.4. Common synthetic routes**

270 Heat Treatment – Conventional and Novel Applications

ii. Chemical energy storage in the form of a fuel;

iv. Mechanical storage in the form of kinetic energy.

iii. Electrical energy storage in the form of charge separation; and

Each of these types of energy storage has its own set of performance characteristics in terms of the energy and power that they can deliver, ultimately meaning that they will be best suited in specific applications. Of course cyclability is also a key performance

Chemical energy, stored as a fuel, can deliver very high specific power and energy, with reasonable efficiency, particularly when it is used in an internal combustion engine [3]. However, such a combustion reaction does little to abate the demand for fossil fuels (which are used most commonly in this domain) or the contributions to greenhouse gas

Chemical energy storage has an added advantage in the sense that it can also be released electrochemically (rather than thermally), through devices such as batteries, supercapacitors and fuel cells [4]. While the specific energy and power performance of these devices is much less than that of the internal combustion engine, their efficiency is much higher, in some instances approaching 100%, meaning that they ultimately utilize any fuel

There are many battery systems available for consumer use, and over the years since commercial introduction, their performance has improved due to a combination of advances in material design and cell engineering [5]. The various battery chemistries that are available can be categorized as either primary (single use) or secondary (rechargeable) systems. The battery system that is currently receiving the most attention in the literature is based on the Li-ion chemistry. Here Li+ ions are reversibly shuttled between the positive and negative electrodes in the cell, which with the use of an appropriate non-aqueous electrolyte, can achieve a potential of over 4 V [5,6]. Some of the commonly found materials in Li-ion

i. Negative electrode materials: Li metal, carbon, Li4Ti5O12, Sn and its alloys, Si and its

ii. Electrolytes: organic carbonate mixtures; e.g., 1:1 ethylene carbonate:dimethyl

iii. Positive electrode materials: LiMO2 (where M is a combination of Ni, Mn and/or Co),

Whatever the combination of positive and negative electrode that is used, the performance of the resultant cell is quite significantly determined by the properties of the electroactive materials used, which in turn are determined by the way in which they were prepared.

i. Thermal energy storage as heat;

characteristic.

emissions.

much better.

batteries include:

alloys;

**1.3. Batteries and battery materials** 

carbonate, ionic liquids; and

LiMn2O4 (and doped varieties thereof), LiFePO4, MnO2.

Many different synthetic routes have been used to prepare the materials mentioned in the previous section, so much so that to list them here would be excessive (note the recent review in reference [6]). Nevertheless, the more common approaches can be categorized as being based on either (i) thermal methods, (ii) solvothermal methods, (iii) mechanical methods, and (iv) electrochemical methods. It is also reasonably common to find that a combination of these methods has been used to produce the resultant material. As an example, LiFePO4 can be made by first using a solvothermal process to intimately mix the precursors, with the resultant mixture then subjected to a thermal treatment to make the final product [7]. Overall, the majority of the positive electroactive materials listed above involve a thermal processing step as the last step in their synthesis.

## **1.5. Pitfalls of thermal processing methods**

The overall objective of any synthesis method is to obtain the final product in the desired form for immediate use. This is particularly true for thermal synthesis methods where there is a delicate balance between heat treatment temperature and duration so as to produce the desired material. Of course the choice of these thermal parameters is also dependent on the effectiveness of precursor mixing, with various solvothermal methods being used to ensure appropriate mixing on the molecular level. Contrast this with some of the initial solid state mixing methods (grinding) used in some synthetic efforts, and the implications it has on the thermal conditions necessary [8].

Let us begin by assuming that we have sufficient mixing of our precursors, since the focus of the discussion here is on the actual thermal conditions to be used. Under these circumstances if we were to thermally treat this mixture the temperature and duration of heat treatment would determine the phase purity and crystallinity of the resultant material. Of course a higher heat treatment temperature, and a longer heat treatment duration would ensure phase purity, as well as lead to a more crystalline material. The question at this time then becomes: What is the preferred material crystallinity?

Many positive electroactive materials in Li-ion batteries require very small crystallite sizes so as to minimize Li+ ion diffusion paths, which is commonly regarded as a key limiting factor in performance [7]. Therefore, excessive heat treatment temperatures and durations, while they may ensure phase purity, also lead to excessive crystallization, which is detrimental. Additionally from a commercial perspective, excessive material heating leads to a waste of energy, which can be costly. What is required, therefore, is a method for predicting the optimum heat treatment temperature and duration so as to ensure phase purity and small crystallite size.

## **1.6. This work**

What we will describe here in this chapter is a method for identifying the optimum thermal synthesis conditions, and then explore their effects on the resultant material properties. The system we will use to demonstrate the approach is the heat treatment process used to remove water from the structure of γ-MnO2 prior to its use in non-aqueous Li-MnO2 cells. While this will be used as a representative example, the general method is applicable to any other thermally based synthetic method.

Monitoring the Effects of Thermal

Treatment on Properties and Performance During Battery Material Synthesis 273

The powder was then suspended in ~500 mL of DI water and its pH again adjusted to 7 with the further addition of 0.1 M NaOH. When the pH had stabilized, the suspension was filtered and the collected solids dried at 110°C. When dry the powdered EMD was removed from the oven, allowed to cool to ambient temperature in a dessicator and then transferred

TG analysis was conducted using a Perkin Elmer Diamond TG/DTG controlled by Pyris software. Approximately 10 mg of γ-MnO2 sample was added to an aluminium sample pan and placed into the analyser. The same mass of α-Al2O3 in a similar aluminium pan was used as the reference material for DTA measurements. With the sample and reference materials loaded, and the furnace closed, dry nitrogen gas was passed over the sample at 20 mL/min for 30 minutes prior and during the heating profile. The heating profile applied to

The structure of the materials generated in this work were identified by X-ray diffraction using a Phillips 1710 diffractometer equipped with a Cu Kα radiation source (λ=1.5418 Å) and operated at 40 kV and 30 mA. The scan range was from 10° to 80° 2θ, with a step size of 0.05° and a count time of 2.5 s. Analysis of the diffraction patterns was carried out by fitting a Lorentzian lineshape to individual peaks, with the fitting parameters then used to calculate structural properties such as the fraction of pyrolusite (Pr; as described by Chabre

Morphology was examined by gas adsorption using a Micromeritics ASAP 2020 Surface Area and Porosity Analyser. A representative 0.10 g sample of the manganese dioxide material was degassed under vacuum at 110°C for 2 h prior to analysis. An adsorption isotherm was then determined over the partial pressure (P/P0) range of 10-7-1 using N2 gas as the adsorbate at 77 K. The specific surface area was extracted from the gas adsorption data using the linearized BET isotherm [10] in the range 0.05<P/P0<0.30, while the pore size distribution was determined using a Density Functional Theory-based approach

The composition of the materials was determined using two consecutive potentiometic titrations (Pt indicator and SCE reference electrode) as outlined in Vogel [11]. A blank titration was carried out first in which 10 mL of acidified 0.25 M ferrous ammonium sulfate (NH4FeSO4.6H2O, BDH Chemicals, 99%) was titrated with a standardised 0.03 M potassium permanganate solution (KMnO4, Ajax Finechem, 99%; standardized using the oxalate method [11]), and the volume of permanganate added to reach the end point denoted as V0. Sample analysis was conducted by digesting 0.050 g of the manganese dioxide being studied into another 10 mL aliquot of the acidified 0.25 M ferrous ammonium sulfate solution. After complete dissolution the resultant solution was then titrated using the same 0.03 M permanganate solution, with the volume to reach the end point recorded as V1. For this

the sample was essentially a linear ramp at rates ranging from 0.25°-10°C/min.

to an airtight container for storage.

**2.3. Material characterization** 

(Micromeritics DFTPlus V2.00).

and Pannetier [9]) and the unit cell parameters.

**2.2. Thermogravimetric (TG) analysis** 

## **2. Methods**

### **2.1. Preparation of starting material**

The sample of γ-MnO2 used in this work was prepared by anodic electrodeposition, and hence given the designation electrolytic manganese dioxide, or EMD. The cell used for electrolysis was based on a temperature controlled 2 L glass beaker in which two 144 cm2 (72 cm2 on either side) titanium sheets were used as the anode substrate, and three similarly sized copper sheets were used as the cathode substrate. The electrodes were arranged alternately so that each anode was surrounded on both sides by a cathode. The electrolyte was an aqueous mixture of 1.0 M MnSO4 and 0.25 M H2SO4 maintained at 97°C. Electrodeposition of the manganese dioxide was conducted with an anodic current density of 65 A/m2 according to the reactions:

$$\text{Anode (Ti): }\text{ Mn}^{2+} + 2\text{H}\_2\text{O} \xrightarrow{\text{ } \text{MnO}\_2 + 4\text{H}^+ + 2\text{e}^-} \tag{1}$$

$$\text{Cathode (Cu):}\quad \mathsf{2H}^{+} + \mathsf{2e}^{-} \longrightarrow \mathsf{H}\_{2} \tag{2}$$

$$\text{Overall:}\,\text{Mn}^{2+} + 2\text{H}\_2\text{O} \xrightarrow{\text{H}\_2\text{O}} \text{MnO}\_2 + 2\text{H}^+ + \text{H}\_2 \tag{3}$$

The overall process was carried out for three days, during which time the electrolyte Mn2+ concentration was of course depleted, while the H+ concentration increased. To counteract this, and hence maintain a constant electrolyte concentration over the duration of the deposition, a concentrated (1.5 M) MnSO4 solution was added continually at a suitable rate to replenish Mn2+ and dilute any excess H2SO4 produced. Under these conditions control of the solution conditions was typically maintained to within ±2%.

After deposition was complete, the solid EMD deposit was mechanically removed from the anode and broken into chunks ~0.5 cm in diameter, and then immersed in 500 mL DI water to assist in the removal of entrained plating electrolyte. The pH of this chunk suspension was adjusted to pH 7 with the addition of 0.1 M NaOH. After ~24 h at a pH of 7 the suspension was filtered and the chunks then dried at 110°C. After drying the chunks were then milled to a -105 μm powder (mean particle size ~45 μm) using an orbital zirconia mill. The powder was then suspended in ~500 mL of DI water and its pH again adjusted to 7 with the further addition of 0.1 M NaOH. When the pH had stabilized, the suspension was filtered and the collected solids dried at 110°C. When dry the powdered EMD was removed from the oven, allowed to cool to ambient temperature in a dessicator and then transferred to an airtight container for storage.
