The increasing share of inverter-based resources in bulk power systems reduces the total amount of mechanical inertia and increases the stochastic uncertainties of power system generation; both factors could cause the future power grid to operate closer to stability limits. Meanwhile, the need for flexible operations will cause more disturbances in future power grids which will challenge the transient stability of the system. To cope with the challenges, this paper proposes a novel wide-area control method utilizing battery energy storage systems to improve the transient angular stability of synchronous machines. The proposed method does not require information of the post-disturbance system and is robust to dynamic modeling uncertainties. The effectiveness of the proposed method is shown theoretically via transient energy analysis and demonstrated through simulation results in a multi-machine test system. Moreover, its advantage over a local control method is demonstrated and discussed.