Timber splits most easily when still green, it will be harder work if it has dried out. Large or awkward shapes are best split using 'log wedges' and a maul or sledge hammer, rather than a splitting maul. Experience tells me to have at least 3 wedges to avoid getting one stuck. I have always used traditional wedges, rather than the star pointed 'log grenade'.

However the wedges are now only a back up to a mechanical splitter. I use a small hy-crack (www.hydrocut.co.uk) which will split a 15" long log of any diameter that I can lift. The wedges are kept for those bits too heavy to lift.
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Simonridout